Add translation project promotion link (#12736)

This commit adds promotional notice on appearance settings about
translation project if any other locale than English is used. It
allows users to learn and contribute translations to Mastodon.

Step ahead, in this commit one unusual string is added - link to a
guide. By default it refers to Crowdin project itself, but if any of
Mastodon localization teams established their own guide, they can
refer it. Or, if Crowdin supports localized domain for language, it
can also be put there (e.g. https://fr.crowdin.com/...).