On my system with a locale of fr_CH, mcomix doesn't find the locale file. I thinh this is because that file is installed into the main location for the language when, mcomix look for that file into the user locale location, which can include a possible variant.
# ls /usr/lib/python3.7/site-packages/mcomix/messages/fr/LC_MESSAGES mcomix.mo
$ mcomix 14:28:17 [MainThread] ERROR: locale file: /usr/lib/python3.7/site-packages/mcomix/messages/fr_CH/LC_MESSAGES/mcomix.mo not found.
If I make a symlink fr_CH-> fr in /usr/lib/python3.7/site-packages/mcomix/messages/, mcomix work fine in French.
I think, as it can be many variantes, the right solution is to make mcomix to simplify the user locale when it contain a variante.
Last edit: Dominique Michel 2021-04-23
I think using Python 3 has fixed this issue, since the translation domain now seems to be simplified as desired: 'fr_CH.UTF-8' > 'fr_CH' > 'fr.UTF-8' > 'fr'