Thank you for looking into this and for the effort of recording a video. The only thing I was missing was some music. That's what were all here for, isn't it? ;-)

It is really strange that for you this appears in v3.2 already. So, to repro this, I set up ru-RU on my computer. Much to my surprise I got yet another behavior, one that I would have least expected: Musescore stayed in English! To be sure, I even rebooted, but it was still English. The only thing that was not set to Russian was 
Язык голосовых функций; because I hadn't installed that functionality. That was still set to German, so it seems even more unlikely that that would set the UI to English.

There was one possible collateral damage I did to my system: Right after reboot I was asked the dialog attached as #### . Since at that time Windows was not running yet, I had no recourse but my very rusty knowledge of Russian. I decided to select пока пропустить - so what on earth did I do?

To fully repro the behavior your video shows, I would have to know what you set for "Язык интерфейса Windows" in the Язык dialog. I kept "Русский"; is that what you have, too? Either way, it seems very odd that I don't get ru-RU if I set everything to that locale. But Windows is always good for surprises like that. 

I also checked the Musescore Resource Manager (something I didn't know existed until today), and it says: Русский : locale_ru.zip : 692.62 KB : Updated (button grayed out). So that couldn't be it, either. 

Of course, there's the difference that I'm still using 3.4. But if I understand you correctly, you already tried that. (Anyway, that seems like an unlikely explanation, since the version in between would have to behave differently from those before and after.)

Of course, for professional software development we should first make sure that we have the exact same test environment. But my computer isn't set up for that. Is there maybe a remote login available for that? 

Failing this, all I can think of is that you could try it with the same locales as I did, but frankly, my hopes aren't high that that would help; most likely you get the same result as before. 

At least I can answer your question: I see the expected behavior already in Блокнот, as well as in the regional settings dialog itself, as you can see from the attached screenshot. Both remain in Russian, even after setting Текущий формат to Английский (США).