Support testing against non-en locales

[zenmonkeykstop]

Install-time language settings affect things like user directory creation (see freedomofpress/securedrop-workstation#1148 for an example). It would be nice to be able to have different-language Qubes snapshots available (since some changes get baked in at install), along with a way to specify which to use for a given test run (the existing yaml file might work).

[legoktm]

According to https://wiki.archlinux.org/title/XDG_user_directories we can change the directory layout by running something like:

LC_ALL=fr_FR.UTF-8 xdg-user-dirs-update --force

We should also change the system locale.