Allow to change the UI language via a setting on macOS

[kelson42]

Follow #603 (comment).

Remark: on iOS this is already handled in the system (but app related) settings

[kelson42]

@BPerlakiH This can be done right now

[rgaudin]

This is very rare to my experience on proper macOS apps. I think the guidelines are to only ship a localized version if the localization is complete or complete-enough (this would need to be defined).

I haven't read the ticket but from its title, it might be about the same topic: openzim/zimit-frontend#72

[BPerlakiH]

To me the openzim ticket is about something we've already solved here: #1089
Btw, I've let them know that we already have a solution for the same issue, they might gain some inspiration from our solution.

Back to our topic, I do agree, it's a very rare case, I have not seen any macOS application that would allow me to change the UI language of that single app.

[yannikbloscheck]

Since a few major version on macOS there is also a system setting to change the language of any specific app like there is on iOS. It's just at a different location on macOS. The setting can be found in the system settings under General/Language & Region. There at the end is a list of applications to which the user can add almost every app with a chosen custom language (as long as that language is generally supported by the app).
So I'm not sure a custom language selector in the app itself really would be necessary on macOS as that already is possible trough the system (even if more hidden).