Are you describing something happening right now, posessively (i.e. right in front of you?) Then the sentence "they are stopping in front of the traffic lights" is correct.
But are you describing a general activity, which you have observed, and which you do not currently possess, in front of you, at the moment? In which case referring to the definite possessive form "they are" is incorrect - who are 'they'? Are you observing this activity immediately?
The correct way to say this sentence: "Germans stop at traffic lights." They're not doing it right now, they're not in front of you - its a general case.
There is a reason to understand and use language properly. I happen to live in a German-speaking country, and this mistake is made often because it is, simply, not taught properly. (I also get my German corrected in the same way, and OFTEN, so lest you feel I'm being curmudgeonly for any reason other than revenge .. ;)
