GL Menu does not allow changing language

[leezer3]

https://github.com/leezer3/OpenBVE/pull/972

As part of doing various bits of testing for this PR, I've noticed that the GL3 main menu (primarily for Mac) doesn't support changing the language. This is an oversight, and should be changed ASAP, although it'll probably require an entirely new menu type.

Only thing that may be difficult is going to be the flag image, but that could probably be skipped for the minute.