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 f

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.