On my forum-index, the last posts in a forum are displayed with last post dates. When i enable translation keys, i get '_date_this_week_c', which only shows me;

<word key="_date_this_week_c" js="0">%s at %s</word>

The same result for the sidebar topic-lists;

<word key="topic_started_date" js="0">Started %s</word>

The wildcards show daynames that aren't translated, after translating the day_0, day_1, day_2 etc. keys, and the MO,TU,WE etc. keys. 

Am i missing some translations?

The stupid idea of taking dates from the webserver locale results in this. 

We pull the date from the locale (which is, in fact, not at all a "stupid" idea), which means if you need to show the date in a different language, you should adjust the locale used for your language pack (or install a different language pack, etc.).