Could you be forced to use a particular language by law?

Today, I saw interesting news on http://www.euronews.com/2014/12/08/merkel-distances-itself-from-speak-german-at-home-idea/

 

This report is about some German politician who want to introduce a law which states that everybody in Germany has to speak German at home. 

The first reaction to this idea was my williness to understand of how they would controll it and how such proposals are possible.