Just found transifex account from years ago as I was translating the early version of V4. I did a few changes so I don’t have to edit texts.php on every update. However, if you apply my changes in your next update, it might break something for others. I don’t think this is good. Don’t know how this goes anymore.
Anyway, my suggestion is to have a texts_custom.php with only custom lines. If the line is not found there, InvoiceNinja uses texts.php
Or if GUI is to be used (even better for hosted users), to add language for each Custom Label defined. That would solve many multi-language use issues. Some users might use more than one language simultaneously for domestic and foreign clients. That would solve most of multi-language requests.