Translations Builder is an external tool created for Power BI Desktop specifically to assist report authors and dataset authors with tasks associated with creating translations and building multi-language reports. As a user, you can install Translations Builder and use it together with Power BI Desktop to build and test datasets and reports that support multiple languages. As a developer, you can clone the GitHub repository with the Translations Builder source code and extend this application to meet whatever translation and localization requirements your organization faces.
Here are the primary links for learning how to build multi-language reports for Power BI using Translations Builder.