Synchronise core language files

[th-h]

The language files in core are a bit out of sync - different order, duplicate definitions, some new constants not added to every language, some old ones not deleted everywhere.

• Add a check script to find and report those deviations
• Remove duplicates in all language files
• Add missing constants and remove those that are no longer present in EN (and no longer used in code)

[stephanbrunker]

I have done this, because I added a few additional definitions for my new feature branch. I have sorted the english file according to the file/module where the strings are used, put all these where I couldn't find a usage into a separate "orphaned" section and then wrote an langsorter.php script which ordered all the files according to the english one.

Already included in the feature_subscription branch, but you can cherry-pick the commits c773b5f and fb928af which handle the language files.

Remaining Todo is then to check the orphaned constants - I found 73 from line 1203 to 1279.