Change available languages according to the topic #196

Issue #196 

This PR changes the langs options in the translation directive.
If the current lang is not found in the topic langs -> fallback to de

We have some missing infos in the BOD (background layers definition are missing for all the topics but blw, inspire and ech) @gjn you can add the missing projects for the layers pixel-farbe, pixel-grau and swissimage in column projekte of the table dataset in bod.