Bug #1263, accept IETF language tags - internationalization/i18n

* Plus, allow language/ language pack to be set semi-dynamically
* Relates to, #1210 - maybe?
* https://github.com/johndyer/mediaelement/issues/1262
Bug #1262, Make the play/pause button more accessible

* Separate "Play" and "Pause" text, depending on current state
* TODO: adjust translations
Bug #1262/ #1263, Separating "Play" & "Pause" in language Javascripts

* Plus, removing language-translation JS from Gruntfile.js
Merge pull request #1270 from IET-OU/iet-ou/CR1262/a11y

Accept IETF language tags, plus accessible play/pause button -- "iet-ou/cr1262/a11y"