Suggest the reader to translate a document to a locale without translations

I just had a passing thought: it might be a nice touch to include all the unfrozen locales in the language drop down list, but gray out the ones that have no translation. If the user clicks on a greyed-out language (or hovers over it), it should display a message requesting the reader's help to translate the doc and provide the links to get started.

To add to that, perhaps we could add an additional entry, "Add your language", that links them to the contributor docs about how to add their own language to MDN (and the work one would need to commit to in doing so). Or a poll to determine how much interest there is to add a new locale, since a user's preferred language may not be interest to get localization in that language.

Start at the language dropdown entry point and inspect how unfrozen locales and translation availability are represented. Define the interaction and link targets described in the issue: untranslated locales should explain how to help, and an “Add your language” entry should point to contributor guidance; verify the dropdown behavior and links.