get language list in l10n library

Let's create all lists of languages centrally in localization library. This harmonizes inconsistencies and removes duplicated code. The list has the following features:

* returns languagename and region
* supports RTL languages
* supports returning the names in that very language OR in one chosen by user
* sorted by language name

This supersedes: https://github.com/concrete5/concrete5/pull/1318