Yes, sure, immigrants should swear an oath to the constitution. Not every German has done that - except if they're a civil servant or served in the Bundeswehr - but you can certainly demand it of immigrants. Because terrorists would never swear falsely and would be completely frightened away by it anyway and run off.
And when it comes to German language skills: we still have Bavaria in the Federal Republic, don't we?
At tagesschau.de - Die Nachrichten der ARD there's the original article.