Thanks for the reply, Traumflug.
Yes, it is on Windows, but I thought xampp was supposed to take care of intrinsic differences like directory structure, separators, etc. I'd try it in a subdomain online, but I'd have to persuade my hosting company to make Thirty Bees available and that seems like a lot of hassle just to test it.
Something I discovered whilst waiting for this post's approval was that xampp actually uses MariaDB rather than MySQL – even though their config pages still refer to it as MySQL – and I wondered if that might have something to do with it. Although they reckon they're supposed to be structurally identical, given that MariaDB is a fork of MySQL.
As for the language thing: well I just wanted to ensure the front office didn't have the choice block and it seemed to make sense to add British English and make it the default at the same time. I'm not really bothered about US spelling in the back office.