Duke3D: Add language option to the "Display Settings" menu
This provides menu access to the locale setting that is right now only controllable through the "locale" console command.
I decided to use "Display Settings" because "Game Settings" are hidden in Fury.
Additionally:
- Selected locale is saved inside the eduke32 config file.
- Default language is set to "en" on launch, i.e. English.
I noticed that localization has been part of the engine for a while, but in Duke3D there hasn't been a way to access it for normal users, making it a very obscure feature. This change could make the possibility more obvious and more easily accessible.
Suggestions and concerns appreciated as always.