I know flatpak is not default in UBudgie 18.04.
It’s not the first time I try to test gimp 2.10.x through snap and flatpak and each time I met the same problem : Gimp 2.10.x sticks to english language whatever I set in its preferences.
It’s the first time I test on my actual UBudgie 18.04 system, so it’s a fresh install of flatpak ( from ppa, no other choice in 18.04 ) and once again, gimp desperatly stays in english whereas my system language is french, even if I choose french in preferences.
So if you have any hints and tips…
On a side note, global-menu applet does not work for gimp flatpak.
To fix it just run flatpak update and after you’ll be able to choose your favorite language in Gimp’s preference.