Minigalaxy crashes on startup due to unsuppoted locale

Title says it all, but for completeness, Minigalaxy crashes on start if the user has a non `en_US.UTF` or `C` as the `LANG` environment variable.

**Stack trace**
```
khalil:~ % flatpak run io.github.sharkwouter.Minigalaxy

(minigalaxy:2): Gtk-WARNING **: 20:12:59.150: Locale not supported by C library.
Using the fallback 'C' locale.
Traceback (most recent call last):
File "/app/bin/minigalaxy", line 66, in
main()
File "/app/bin/minigalaxy", line 57, in main
from minigalaxy.ui import Window
File "/app/lib/python3.7/site-packages/minigalaxy/ui/__init__.py", line 2, in
from minigalaxy.ui.window import Window
File "/app/lib/python3.7/site-packages/minigalaxy/ui/window.py", line 5, in
from minigalaxy.ui.login import Login
File "/app/lib/python3.7/site-packages/minigalaxy/ui/login.py", line 8, in
from minigalaxy.translation import _
File "/app/lib/python3.7/site-packages/minigalaxy/translation.py", line 7, in
locale.setlocale(locale.LC_ALL, '')
File "/usr/lib/python3.7/locale.py", line 604, in setlocale
return _setlocale(category, locale)
locale.Error: unsupported locale setting
```
**Workaround**
```
flatpak --user override --env=LANG=C
```
I'm not sure if this is a flatpak packaging bug (like Discord crashing when the timezone is different), or an upstream (the non-flatpak Minigalaxy) one, if this is the wrong place to report this just say it =p

Fell free to ask any further logs/questions if needed.