To assign a language to a project member, navigate to the Contributors. In the dropdown menu, select the “Edit language permissions” option. This will open a modal window where you can select all the languages the person will be allowed to translate.
Once assigned, users will only see their designated languages in the language list. However, they will still be able to view a list of translations in other languages (in read-only mode) in the Languages tab while translating. This helps them understand the context better.
Read more about language permission feature in documentation.