i18n improvements

Features added / changed:

- i18n support for [ISO 693_1](http://en.wikipedia.org/wiki/ISO_639-1)
- i18n.getLanguage() Internet Explorer language detection
- i18n.t() converts special chars to html entities (like mejs.Utility.escapeHTML)
- commented lines, fixed some typos
- locale files (german, traditional chinese) check if the langcode is not already set

The last point is related to pull request [850](https://github.com/johndyer/mediaelement/pull/850) and [940](https://github.com/johndyer/mediaelement/pull/940), both targeting i18n + Wordpress. I think checking prerequisites is a good idea, therefore my own locale files do not overwrite existing locale objects anymore.

You have to run Builder.py manually to create the *min.js files. No Java on my machine :)