Hi there.
In 120.2.2 r8162M (version downloaded via Steam) every time I change the language to French in the GUI, it changes for Arpitaan (both Arpitan and French boxes are checked...)
I also tried for changing the language in settings.xml and it has the same effect.
I'm on Fedora 26 (linux).
delete simutrans/text/frp.tab
or rename to fp.tab
Sorry, this issue will be fixed in the next version.