I already asked it here, on stackoverflow and several other services. But nobody can give me an answer for this.
The DRF documentation says the following:
If you're only translating custom error messages that exist inside your project codebase you don't need to copy the REST framework source django.po file into a LOCALE_PATHS folder, and can instead simply run Django's standard makemessages process.
So, it works. But after that all DRF error messages are not translated as they were by default.
I don't want to translate the whole DRF .po file. I want to translate ONLY my custom error messages and also I want the default DRF error messages stay translated.
Just in case here's the link to my question on stackoverflow