Comment 12 for bug 1576411

for the non bindtextdomain snaps (i.e. any binaries that just use libc's locale functions)
line 7-21 in
http://bazaar.launchpad.net/~ogra/+junk/nethack/view/head:/nethack.sh

plus the libc-bin and locales packages in "stage-packages" are a solution if you want to ship your own locales inside the snap (this should work fine for most IoT use cases)

this is currently also used by the freeCAD snap in the store.

i assume for desktop use we actually need an interface, though since the locale data itself will be inside the app snap while the locales command will live in the core snap or in the desktop install itself this will likely require some extra work and some bind mounting magic to make the two come together.