Locale descriptions in switching language modal.

Here i take up the pull request https://github.com/adobe/brackets/pull/3677.

I added english i18n for the locales and i moved them in the strings.js to the modal strings.

additional infos:
issue https://github.com/adobe/brackets/issues/1461
card https://trello.com/c/Pjw3vGt2

I also removed the locale label string, because a short descriptions "feels" a lot more usable than a locale in ( ). So now there will be plain "English" instead of "en". And because of the fallback there wont be a plain locale in the modal anymore.