Ensure the GTK / libadwaita language is consistent with Pinta

[cameronwhite]

This was mainly an issue on macOS where the system language setting is picked up correctly by dotnet (setting the current UI culture), but the GTK / libadwaita translations from the native gettext library were still in English

This change also works toward having a preference to override the default language (#1417)

Fixes: #1727

[cameronwhite]

This worked locally when I built the mac installer on my system, but not when using the installer from the CI build. I suspect the issue is that the native libraries like libadwaita aren't perfectly relocatable and have references to the install path they were built for (e.g. https://docs.gtk.org/glib/i18n.html#internationalization). We might need to call bindtextdomain() manually to override the location, although that feels hacky...