Switching language is not applied everywhere until restart
This only applies to application windows (the main interface seems to switch language successfully as far as i can see) - and it comes in two flavours: (a) some windows always have the issue and (b) some only they were opened at least once before switching language.
I guess (b) indicates that the i18n strings are cached in memory when opening such windows and those then don't get invalided properly when switching language.
Steps to reproduce:
1. Start MuseScore with whatever language it's set to.
2. Go to "Edit → Preferences..." and change current language [A] to whatever you like [B] (i discovered this when changing from "System" to "English (US)" but i works vice versa and with other languages too) and hit "OK". The main GUI now shows in language [B].
3. Go to "Plugins → Plugin Manager...". The window title of the plugin manager and all field names still show in Language [A].
The following windows expose the same behaviour, but only if they have been opened before you switch the language:
* "Edit → Instruments...": Only window title still has lang [A], content is [B] though.
* "View → Master Palette...": total mixup, most of the window content still shows in language [A], but window title, as well as 'Key Signatures', 'Time Signatures' and 'Symbols' show as [B].
* "Format → Style...": everything is still in language [A]
All other windows it tested properly show up in [B].
Only after having closed and reopened MuseScore, everything is finally shown as [B].
I know this sounds like a bug so minor not even worth mentioning (given there is an easy workaround), but if i had written that code i would want to know about it asap - as chances are this may point to some underlying issue or structural problem, or at least help learn from it to come up with a more robust caching strategy, thus i'm reporting it here.
Issue discovered in 3.2.4, and i can confirm it still exists on Manjaro Linux, Arch.: x86_64, MuseScore version (64-bit): 3.5.0, revision: 3543170