Curated list of localized language names for Firefox UI

Hi localizers,

Follow-up to the email about dictionaries. Another part of improving the 
experience when switching language is to display localized language 
names in the UI. Currently, Firefox display languages localized in the 
language of the build, for example showing "French" in an English build.

This happens for spell checking in the context menu, when customizing 
the preferred languages for web content, and for the brand new language 
switcher.

Like for dictionaries, we want to create and maintain a curated list of 
language names to use in Firefox UI, instead of exposing hundreds of 
strings to all languages for translation. There are a few challenges, 
and we need your help to understand if we're using the right approach.

This is the initial curated list
https://mozilla-l10n.github.io/firefox-languages/

It's manually generated by looking at CLDR data where possible, since it 
provides both localized names, and a capitalization rule to use such 
name in the context of "UI list or menu", which is exactly the context 
in which these will be used.

Here you can find the full analysis
https://mozilla-l10n.github.io/firefox-languages/complete.html

I realized afterwards that the dictionary discussion generated a bit of 
noise in the mailing list, so let's try a different approach.

If something looks wrong for your language, please provide feedback or 
corrections via any of these methods (whichever is easier for you):
* By commenting in this bug 
https://bugzilla.mozilla.org/show_bug.cgi?id=1481729
* By opening a pull request to edit 
https://github.com/mozilla-l10n/firefox-languages/blob/master/output/languages_curated.json
* By opening an issue on https://github.com/mozilla-l10n/firefox-languages
* By reaching out directly to me, either via email, IRC or Telegram

Francesco