How To Implement Cross-lingual Transfer Learning In 5 Different Ways

by | Sep 22, 2023 | Machine Learning, Natural Language Processing

What is cross-lingual transfer learning?

Cross-lingual transfer learning is a machine learning technique that involves transferring knowledge or models from one language to another, typically to improve the performance of natural language processing (NLP) tasks in a target language for which there may be limited data or resources. This approach has gained significant attention in recent years, as it can help address the problem of data scarcity for many languages.

Top 5 methods of implementing cross-lingual transfer learning

Here are the top 5 methods of implementing cross-lingual transfer learning:

  1. Multilingual Models: One common approach is to train multilingual language models to understand and generate text in multiple languages. These models are pre-trained on large, diverse corpora containing text from many languages. Examples include models like mBERT (Multilingual BERT), XLM-R (Cross-lingual Language Model), and mT5 (Multilingual T5).
  2. Cross-lingual Embeddings: Cross-lingual word embeddings are representations of words or phrases that capture semantic similarity and can be shared across multiple languages. Techniques like multilingual word2vec and fastText can be used to learn embeddings that map words from one language to another.
  3. Zero-shot and Few-shot Learning: With cross-lingual models, you can perform zero-shot or few-shot learning, where a model is fine-tuned or adapted for a specific task in a target language with very little labelled data. The knowledge transferred from the source language can help bootstrap the model’s performance in the target language.
  4. Parallel Data and Machine Translation: In some cases, parallel data (text aligned between languages) can train machine translation models. These models can be leveraged for cross-lingual transfer by translating text from the target language to the source language, applying the source language model, and then translating the results back to the target language.
  5. Cross-lingual Pre-training: Similar to how models like BERT are pre-trained on large text corpora in English, cross-lingual models are pre-trained on multilingual corpora. They learn to predict masked words or other linguistic tasks in multiple languages, which helps them capture cross-lingual knowledge.

We will now examine each of these methods in more detail.

Multilingual models for cross-lingual transfer learning

Multilingual models are artificial intelligence models, often based on deep learning techniques, designed to understand and work with multiple languages. These models are trained on diverse, multilingual datasets. They can process text or speech in various languages, making them versatile and useful for numerous natural language processing (NLP) tasks.

Key aspects and characteristics of multilingual models

  1. Pre-training on Multilingual Corpora: Multilingual models are typically pre-trained on large corpora of text from multiple languages. During pre-training, the model learns to understand these languages’ linguistic patterns, syntax, and semantics. This pre-training phase is similar to how monolingual models like BERT are trained but involves a mixture of languages.
  2. Language Agnosticism: Multilingual models are designed to be language-agnostic to a certain extent. They do not rely on hard-coded language-specific rules or features. Instead, they learn to encode text in a language-agnostic manner, allowing them to generalize across languages.
  3. Cross-lingual Transfer: One of the primary benefits of multilingual models is their ability to transfer knowledge across languages. They can be fine-tuned for specific NLP tasks in various languages, even when limited training data is available for a particular language. This transfer learning capability is essential for low-resource languages.
  4. Versatile NLP Tasks: Multilingual models can be applied to various NLP tasks, including text classification, sentiment analysis, named entity recognition, machine translation, question-answering, and more. Their versatility makes them valuable for building multilingual applications.
  5. Zero-shot and Few-shot Learning: Multilingual models can perform zero-shot learning, making predictions in languages they were not explicitly trained on. They can also perform few-shot learning, adapting to a new language or task with minimal examples.
  6. Resource Efficiency: By leveraging a single multilingual model, developers can save resources compared to training separate models for each language of interest. This is especially important for languages with limited available data.

Examples of popular multilingual models

  1. mBERT (Multilingual BERT): A multilingual version of the BERT model, pre-trained on text from over 100 languages. It has been widely used for multilingual NLP tasks.
  2. XLM-R (Cross-lingual Language Model): An extension of the RoBERTa model, XLM-R is trained on a massive multilingual corpus and excels in cross-lingual tasks.
  3. mT5 (Multilingual T5): Building on the T5 model architecture, mT5 is trained to perform various NLP tasks across many languages.

Applications of multilingual models

  • Building chatbots that can communicate in multiple languages.
  • Sentiment analysis across diverse language sources.
  • Machine translation systems that can handle a wide array of language pairs.
chatbots are commonly used for Cross-lingual Transfer Learning

Multilingual models have become a vital component of multilingual and cross-lingual natural language processing, enabling more inclusive and accessible AI applications across the globe.

Cross-lingual embeddings for cross-lingual transfer learning

Cross-lingual embeddings, also known as cross-lingual word embeddings or multilingual embeddings, are a type of representation that allows words or phrases from different languages to be mapped into a shared vector space. These embeddings are particularly useful for multilingual natural language processing (NLP) tasks, as they enable the transfer of knowledge and information between languages. Here’s a closer look at cross-lingual embeddings:

  1. Shared Vector Space: Cross-lingual embeddings are trained to map words or phrases with similar meanings in different languages to nearby points in the shared vector space. This enables alignment between languages, making it possible to perform cross-lingual tasks.
  2. Training Data: These embeddings are typically learned from large multilingual corpora, which consist of text data from various languages. During training, the model learns to predict the context of words or phrases in multiple languages, effectively capturing the semantic relationships between words across languages.


  1. Cross-Lingual Transfer: Cross-lingual transfer learning is one of the primary applications of cross-lingual embeddings. You can use embeddings from one language to improve the performance of NLP tasks in another language, even when there is limited or no labelled data for the target language.
  2. Cross-Lingual Information Retrieval: Cross-lingual embeddings can retrieve documents or information written in different languages. Queries in one language can be matched with documents in another based on the shared vector space.
  3. Machine Translation: Cross-lingual embeddings can be helpful in machine translation tasks. They can help improve the translation quality by aligning words or phrases in the source and target languages.
  4. Named Entity Recognition (NER): Cross-lingual embeddings can assist in recognizing named entities (e.g., names of people, places, and organizations) in text across multiple languages.

Methods for Learning Cross-Lingual Embeddings

  1. Bi/multilingual Skip-gram: These methods extend the skip-gram model (used for word embeddings like Word2Vec) to simultaneously learn embeddings for words in multiple languages.
  2. FastText: FastText is an extension of Word2Vec that can learn embeddings for subword units (character n-grams), making it suitable for handling morphologically rich languages.
  3. Cross-lingual Embedding Mapping: After learning monolingual embeddings for multiple languages, techniques like Canonical Correlation Analysis (CCA) or Procrustes analysis can map these embeddings into a shared space.


  1. Language Divergence: Not all languages have one-to-one translation equivalents, and linguistic structures can vary significantly across languages. Handling such divergence can be challenging.
  2. Data Imbalance: Some languages may have much less training data available, making it difficult to learn high-quality cross-lingual embeddings for these languages.
  3. Out-of-Vocabulary Words: Handling out-of-vocabulary words or words that do not have clear counterparts in other languages is a common challenge.

Cross-lingual embeddings are a valuable tool in bridging language barriers in NLP tasks, enabling the development of multilingual and cross-lingual applications with reduced data requirements. Researchers continue to work on improving the quality and applicability of these embeddings for a wide range of languages and NLP tasks.

Zero-shot and few-shot learning for cross-lingual transfer learning

Zero-shot and few-shot learning are machine learning techniques that deal with recognizing or classifying objects or data points for which there is limited or no labelled training data. These techniques are handy when dealing with novel or rare classes or adapting models to new tasks with minimal examples.

Zero-Shot Learning (ZSL)

Zero-shot learning refers to the scenario where a model is trained to recognize or classify objects, concepts, or data points it has never seen during training. In other words, it can generalize to unseen classes.

How it works: ZSL relies on attribute vectors or semantic embeddings. Each class or category is associated with a set of semantic attributes that describe its characteristics. The model learns to predict the attributes of a given data point and then assigns it to the class with the most similar attributes, even if that class was not present in the training data.

Applications: ZSL is used in image recognition, natural language processing, and other domains. For instance, image recognition can be used to classify species of animals not seen during training.

Few-Shot Learning

Few-shot learning is an extension of zero-shot learning that deals with situations where a limited number of labelled examples (shots) are available for each class.

How it works: Few-shot learning can be categorized into various approaches, including:

  1. N-shot learning: The model is trained to recognize new classes with N-labeled examples per class. N can be a small number, such as 1, 5, or 10.
  2. K-way learning: The model is trained to distinguish between K different classes, each represented by a few examples.
  3. Meta-learning: The model is trained to learn a good initialization or adaptation process to quickly adapt to new tasks with a few examples. This often involves learning a meta-learner that can generalize from several examples.

Applications: Few-shot learning is used in image classification, object detection, natural language understanding, and other tasks where data for certain classes is scarce.

Relation between Zero-Shot and Few-Shot Learning

Few-shot learning can be considered an extension of zero-shot learning. Zero-shot learning is a case of few-shot learning with zero-labelled examples for new classes.

Zero-shot and few-shot learning aim to address the challenge of recognizing or classifying unseen classes or tasks by learning to generalize from limited data.


Designing effective attribute vectors or embeddings for zero-shot learning can be challenging, as they must capture meaningful class information.

Few-shot learning can be challenging when there is a large domain gap between the training and test data or limited intra-class variation.

In both cases, the choice of a suitable model architecture, loss functions, and training strategies plays a crucial role in the success of these techniques.

Zero-shot and few-shot learning are essential for making machine learning models more adaptable to new and unforeseen tasks, reducing the need for extensive labelled data for every possible class or task. They find applications in various domains, including computer vision, natural language processing, and robotics, where the ability to generalize to new situations is valuable.

Parallel data and machine translation

Parallel data is a crucial resource in machine translation, which involves automatically translating text or content from one language to another. Similar data consists of pairs of sentences or documents in the source language and their corresponding translations in the target language. This data serves as the foundation for training machine translation models, allowing them to learn how to generate accurate and coherent translations.

Here’s how parallel data is used in the context of machine translation:

Training Data for Supervised Learning

Parallel data is the primary training data for supervised machine translation models. These models are trained to predict the target sentence given the source language sentence.

Each pair of sentences in the parallel data serves as a training example. The source sentence is the input, and the target sentence is the expected output. The model learns to map from the source to the target language through training.

Data Preprocessing and Alignment

  • Before training, the parallel data often goes through preprocessing steps to tokenize, clean, and align the sentences. To ensure that each source sentence correctly corresponds to its target language translation, sentence alignment is essential.

Types of Parallel Data

  1. Bilingual Parallel Data: This type consists of sentence pairs in two languages: the source language and the target language—for example, a sentence in English and its translation in Spanish.
  2. Multilingual Parallel Data: Some datasets contain parallel data for multiple languages, allowing for training multilingual translation models that can translate between numerous language pairs.

Challenges with Parallel Data

  1. Data Availability: High-quality parallel data can be scarce, especially for less commonly spoken languages. The availability of similar data impacts the quality of machine translation models.
  2. Domain Specificity: Parallel data may not cover specific domains or industries comprehensively. Domain-specific translation models may require additional specialized data.
  3. Quality and Consistency: The quality and consistency of translations in the parallel data are critical. Inaccurate or inconsistent translations can negatively affect the model’s performance.

Expanding Parallel Data

  • In cases where parallel data is limited, techniques such as back-translation can generate additional synthetic similar data. Back-translation involves translating monolingual data (source language to target language and vice versa) using a pre-trained model. This synthetic data can be used to supplement the original parallel data.

Modern Approaches

Neural machine translation models, such as Transformer-based models and their variants (e.g., BERT, GPT), have significantly improved machine translation performance and have used large parallel datasets for training.

Transfer learning techniques allow models to leverage pre-trained representations on large monolingual corpora before fine-tuning on parallel data, reducing the need for massive amounts of similar data.

Parallel data is fundamental in machine translation because it enables the training of models to understand the relationships between words and phrases in two languages and generate coherent translations. As machine translation technology advances, efforts continue to expand and improve parallel datasets to enhance the quality of translations across a wide range of languages and domains.

Cross-lingual pre-training for cross-lingual transfer learning

Cross-lingual pre-training is a machine learning technique used in natural language processing (NLP) that involves training models on multilingual data to create shared representations of languages. The primary goal of cross-lingual pre-training is to enable models to understand and generate text in multiple languages, even when limited labelled data is available for each language individually. This technique has gained popularity due to its ability to address language diversity and the scarcity of resources for many languages. Here’s how cross-lingual pre-training works:

Data Collection and Preprocessing

  • Multilingual corpora containing text data in multiple languages are collected and preprocessed. These corpora can consist of diverse sources, including websites, books, articles, and more. The data is cleaned and tokenized.

Pre-training on Multilingual Data

Often based on deep learning architectures like Transformers, models are pre-trained on this multilingual data. The pre-training task typically involves language modelling, where the model learns to predict the next word in a sentence based on the context of the preceding words.

During this phase, the model learns the grammar and vocabulary of multiple languages and captures cross-lingual information, such as word and sentence alignments, semantic similarities, and language-agnostic features.

Models like mBERT (Multilingual BERT), XLM-R (Cross-lingual Language Model), and mT5 (Multilingual T5) are examples of popular cross-lingual pre-trained models.

Fine-tuning for Specific Tasks

After pre-training on the multilingual data, the model can be fine-tuned for specific NLP tasks, such as text classification, sentiment analysis, machine translation, or named entity recognition. Fine-tuning involves training the model on task-specific labelled data in the target language.

Fine-tuning allows the model to adapt to the nuances of the target language and specific task while benefiting from the cross-lingual knowledge it gained during pre-training.

Cross-lingual Transfer

The critical advantage of cross-lingual pre-trained models is their ability to perform cross-lingual transfer learning. These models can be applied to languages not part of the fine-tuning data, making them versatile for a wide range of languages.

The cross-lingual transfer enables tasks like zero-shot learning (applying the model to a language it has not seen during fine-tuning) or few-shot learning (adapting the model to a new language with minimal labelled examples).


  • Cross-lingual pre-training is valuable in various applications, including building chatbots that can communicate in multiple languages, analyzing social media data in different languages, and enabling machine translation systems to handle multiple language pairs.


Ensuring high-quality cross-lingual pre-training data can be challenging, requiring diverse and representative text data in multiple languages.

Handling language-specific nuances and low-resource languages remains a challenge, as the amount of available labelled data may be limited for specific languages.

Cross-lingual pre-training has become a fundamental technique in multilingual and cross-lingual NLP, enabling the development of more inclusive and adaptable AI applications across the globe by reducing the dependency on large amounts of language-specific training data. Researchers continue to explore ways to improve the effectiveness of cross-lingual pre-training for a broader range of languages and tasks.

How to implement cross-lingual transfer learning

Implementing cross-lingual transfer learning involves several steps, from acquiring the necessary data to fine-tuning and deploying your model. Here’s a high-level overview of the process:

1. Data Collection and Pre-processing:

  • Multilingual Data: Gather a diverse multilingual dataset that contains text in multiple languages. You can obtain such data from various sources, including websites, books, articles, and public datasets. Ensure that the data is representative of the languages you want to work with.
  • Pre-processing: Pre-process the data by tokenizing, cleaning, and normalizing text. Ensure consistent encoding and handle language-specific issues like character sets, diacritics, and script variations.

2. Choose a Pre-trained Model:

  • Select a pre-trained cross-lingual model that suits your task and the languages you’re interested in. Popular options include mBERT, XLM-R, and mT5. You can usually find pre-trained models and their checkpoints provided by the model creators.

3. Fine-tuning:

  • Fine-tune the pre-trained model on your specific NLP task. Depending on your task (e.g., text classification, sentiment analysis, named entity recognition, translation), you’ll need task-specific labelled data in your target language. Prepare this dataset with the correct labels or annotations.
  • Fine-tuning often involves updating the pre-trained model’s top layers (classification head) while keeping the lower layers frozen. This allows the model to adapt to the task while retaining its cross-lingual knowledge.

4. Cross-lingual Transfer:

  • Once your model is fine-tuned, you can apply it to languages not part of the fine-tuning data. This is where cross-lingual transfer learning comes into play. The model should perform reasonably well in these new languages due to its cross-lingual understanding.
  • For zero-shot or few-shot learning in a new language, provide a few examples in the target language to guide the model’s predictions.

5. Evaluation and Tuning:

  • Evaluate the performance of your model in various languages and tasks. Use appropriate evaluation metrics to assess its effectiveness in cross-lingual scenarios.
  • Fine-tune and adjust hyperparameters to optimize the model’s performance on specific languages or tasks.

6. Deployment:

  • Once satisfied with the performance, deploy your cross-lingual model in your application or service. Ensure that it can handle multiple languages seamlessly.

7. Ongoing Maintenance and Updates:

  • Keep your model up to date with new data and emerging language trends. Fine-tune it periodically to adapt to evolving languages and tasks.

Additional Tips:

  • Utilize resources for low-resource languages: Consider techniques like back-translation to generate synthetic parallel data for fine-tuning languages with limited available data.
  • Handle language-specific nuances: Be aware of language-specific linguistic features and variations, and adjust your model or pre-processing accordingly.
  • Monitor and assess model fairness: Ensure your cross-lingual model provides equitable performance across languages and cultures to avoid bias or discrimination.
  • Collaborate with experts: If possible, collaborate with linguists or experts in the target languages to ensure the quality and appropriateness of your model’s outputs.

Implementing cross-lingual transfer learning can be a complex task, but it can greatly enhance the versatility of your NLP applications and enable them to serve a global audience. Careful data collection, model selection, fine-tuning, and evaluation are key to the success of your cross-lingual NLP project.

Summary of cross-lingual transfer learning

Cross-lingual transfer learning and multilingual models have emerged as powerful techniques in natural language processing (NLP) and machine learning. These approaches address the challenges of language diversity, data scarcity, and the need for versatile NLP applications in a multilingual world. Here are the key takeaways:

Cross-lingual Transfer Learning:

  • Cross-lingual transfer learning involves transferring knowledge or models from one language to another to improve NLP tasks.
  • Techniques like multilingual models, cross-lingual embeddings, and cross-lingual pre-training enable effective cross-lingual transfer.

Multilingual Models:

  • Multilingual models, such as mBERT, XLM-R, and mT5, can understand and generate text in multiple languages.
  • They are pre-trained on diverse, multilingual data and are versatile for various NLP tasks, promoting multilingual AI applications.

Cross-lingual Embeddings:

  • Cross-lingual word embeddings allow words and phrases from different languages to be mapped into a shared vector space, facilitating cross-lingual tasks.

Parallel Data and Machine Translation:

  • Parallel data, consisting of source and target language pairs, is essential for training machine translation models.
  • It enables models to learn the relationships between languages and generate accurate translations.

Zero-shot and Few-shot Learning:

  • Zero-shot learning allows models to recognize unseen classes or tasks, while few-shot learning deals with limited labelled examples.
  • These techniques enhance adaptability to new and rare classes, reducing the need for extensive training data.

Cross-lingual Pre-training:

  • Cross-lingual pre-training involves training models on multilingual data to create shared language representations.
  • These models can be fine-tuned for specific tasks and perform cross-lingual transfer, making them versatile and adaptable.

In a world characterized by linguistic diversity, these cross-lingual and multilingual techniques are pivotal in making AI more inclusive and accessible across languages and cultures. Researchers and developers continue to advance these methods, striving for better cross-lingual understanding and developing AI systems that can seamlessly operate in many languages.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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