Build languages.js

hm, that was my plan for languages (slug to gettext). i see we use that for ```NEWSLETTER_LANGUAGES```. it'd also be prudent to only add languages we need. to save KBs, marketplace-constants should support a canonical list of supported languages and only generate a mapping from that, rather than dump all of mozilla languages. if we only use supported langauges, we can reuse it for things like ```data-languages``` in the packaged template and within ```l10n.js```