Write a regression test for the Unsafe Browser starting in various languages

See #8693 (closed) => we should test it in all cases: locales that have a corresponding language pack with a short locale string, fallback on langpack for locale that starts the same, etc. See the guess_best_tor_browser_locale, and especially the changes introduced to fix #8693 (closed), for the details.

I think it’s probably too much work to implement this test by logging in using various languages: we would need to make many screenshots locale-dependent. We’ll come to it at some point, but for now IMO testing by setting locales manually in the terminal before starting the Unsafe Browser will be good enough.

Feature Branch: kytv:test/8701-unsafe-browser-languages

Parent Task: #8540 (closed)

Original created by @intrigeri on 8701 (Redmine)