True but then I would assume that the ILocalizedTextService would also use this?

The backend still has the embedded resources used for ILocalizedTextService, nothing has changed there. If you want client translations, you will have to register the new translation keys through a manifest file, though. Those can be registered either directly in the manifest file or as a javascript module.

Sure I had those for some parts already but not looking forward to modify all backend code as well, but thanks

Backend code should be the same