Slugs containing "de", "fr" or "it" could be problematic

The new language switch replaces "de", "fr" and "it" in the URL. This could be a problem for urls with those texts in the slug (e.g. "fruit" or "iterations").

Several Options:
 - replace the string with the slashes (e.g. "/de/")
 - document the issue in the contributing file and hope no one adds a slug like this

See also https://github.com/scout-ch/docu/issues/69