Bug 41369: Improve Firefox language settings for multi-lingual packages

Change the language selector to be sorted by language code, rather than
name, and to display the language code to the user.

Bug 41372: Handle Japanese as a special case in preferences on macOS

Japanese is treated in a special way on macOS. However, seeing the
Japanese language tag could be confusing for users, and moreover the
language name is not localized correctly like other langs.

Bug 41378: Tell users that they can change their language at the first start

With multi-lingual builds, Tor Browser matches the user's system
language, but some users might want to change it.
So, we tell them that it is possible, but only once.