run-session: fetch Language from AccountsService and set it if exists

When users set their preferred language through GNOME Settings, the application forwards that information to AccountsService, that stores it under /var/lib, since the preference may vary per user. In pmOS we setup a default language with pmbootsrap, but there's no support for changes after installation, unless the user modifies files under /etc/profile.d Make it possible to set the language graphically in Settings by fetching the appropriate language from AccountsService through dbus, and setting the variable if the language is actually set.

Fixes pmaports#946 (closed)

Depends https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/46718

Marked as Draft since I haven't tested it yet. This adds a busctl depends, but might be worth it?

Also @PureTryOut, any idea of whether plasma is also using AccountsService, and whether this might also fix some localization issues there?