Fix internationalization when using multiple languages

Fixes http://www.tinymce.com/develop/bugtracker_view.php?id=7207

I did not come up with the language code + '.' prefix in `i18n.data`, this is already code used in Editor's translate function, which tries to find a specific language's translation by prefixing the language code and dot. Somewhere along the way, there was a mismatch in expectations of what is stored in `i18n.data` between `Editor.js` and `I18n.js`, and this fixes that.