Introducing language ROADMAP.md

[ezenzen]

Hi @all,

this PR suggests a ROADMAP for cleaning up and streamlining the language files. There are many working groups dedicated to improving the language in ILIAS. The individual sections are coordinated with the respective groups. Adjustments are monitored via the ILIAS issue tracker.

Renaming/unifying language variables should reduce the number of variables in the long run.

Greetings, Fabian Kruse and Enrico Zenzen