[Enhancement] Remember previously used language codes

[nicolas-raoul]

I often mistype. Or mix uppercase/lowercase. Or mix left/right, not knowing whether it is jp_JA or ja_JP.

[callahad]

As a first step, how's this for a dumb auto-complete?

https://jsfiddle.net/5f0rq05r/

[callahad]

Though, unfortunately, auto-completion based on datalists is currently broken in add-ons (Bug 1342708)