First, you are putting the quote marks in the wrong place. To quote a value in searches, the opening quote goes after the colon, as in last_modified:">in diesem monat"
However, even after doing that the searches won't work.
The problem is the capital letters in the translated string. The code (well, OK, I) did not account for this possibility.
I have submitted a fix for the capital letter problem. While there, I also added the possibility to use values that are never translated: _today, _yesterday, _daysago, _thismonth. These changes will be available in the next release of calibre.