This is built in based on a suffix on the end of the transformation's code name. check out the documentation here. Look at the last blue box on the page labeled with: Transformations for multilingual websites
You may need to display different text in transformations based on the currently selected language. If you are using the built-in multilingual support, you can achieve this by creating a separate transformation for each language, using the appropriate culture code as a suffix in the transformation name.
- English (default language) transformation code name: cms.news.detail
- French transformation code name: cms.news.detail_fr-fr
When a user switches the content culture to French, web parts and controls automatically load the French version of the transformation.