Most work was done in 7dc0a05a71d958918644339649ac6874c821eacb, though
`check_translation.rb` still had a hardcoded shebang. This is now
fixed. #2590 is now fully resolved too.
helper-scripts/check_translations.rb checks if keys present in the master
translation file are used in any php or js code. Writes a sed comand to
stdout to remove unused keys from all lang files.
This saves unnecessary work when translating.
