As far as I know this is just a convenience thing so that various apps can query the preferred language - Mac OS itself only supports about 40 or so languages for system translations and third party apps will mostly only support only a few (and one of those will invariably be English). Perhaps it does a few other quality of life things like sets reading order correctly (i.e. RTL or LTR) but your menus won't be in some language that only 2500 people speak.

> macOS has some really exceptional internationalization.

Ubuntu is pretty good on that aspect as well, that's why I prefer it for my parents compared to Windows which is spotty in a lot of places. As an example "Windows Update" isn't translated and does not mean anything in a foreign language.

After reading this I decided to scroll through the available languages and honestly can't believe it has Kernewek (Cornish), a language from the British county of Cornwall. Reading around, estimates put the number of fluent speakers at around 400 and those with basic language skills at around 5,000.

It's System Settings -> Language and Region on my Mac (no General).

I may change my Mac to Latin for fun.