The language will influance indeed only the text on the pages not the UI of the modeler itself. For the documentation see here: https://docs.mendix.com/howto/collaboration-requirements-management/translate-your-app-content#2-adding-app-project-languages
And if you want to make the data multilingual you can find an old but still useable forum post about it: https://forum.mendix.com/link/questions/5419.
Regards,
Ronald