It’s a very general question and I did not find any answers yet. I already tried to overwrite the english texts in the en.js file (in /wwww/src/lib/lang/) for testing and looks like nothing happens. So I guess it reads the /rom folder or it is not the correct file
As we are going to firmware 4.x, we don’t have any plan to translate further. So I don’t have an aswer to your questions.
I mean I would like to do a custom translation on my own in my spare time.