Mypaint definitively needs some tuning about language settings. I downloaded the software, ran it and it was in... Norwegian! My OS is in English, every programs I use are in English. I have no idea where this software got the idea it should use Norwegian.
Looked in the settings: no way to change the language. Looked in forums: nothing worked. Many people said you only need to rename a folder. Believe me, I change the name of every possible folder with no, NO, norway, or whatever that looked like Norwegian.
Nothing helped. It can be that the language was cached somewhere and that the program would have worked in English after a restart of the PC but I found another solution:
1) Uninstall your program (be careful if you already have done some work with it, you may lose all the data)
2) Reinstall it and be careful. At what point, you will be asked what you want to install
3) Simply uncheck "Translations"
4) Enjoy :-)