Multi language / German improvements

This is a follow-up pull-request to #32 , improving and fixing various problems regarding German language support.

**Parsing**

- Adds support for plural add-statements, e.g. ``Ein Redaktor kann neue Inhalte hinzufügen.``
- Fixes a conflict with the keyword ``Inhalte``, which is usually stripped but should not be stripped for the action group ``Inhalte konfigurieren`` - see the commit message body for details.
- Adds multilingual filename support (e.g. ``specification.de.txt``) to the automatic workflow test.
- Fixes some encoding issues regarding translations of action groups.

@maethu @deiferni