Do all Germans yell and scream constantly like these characters?

actually there's just as much screaming and yelling going on in the average hollywood movie as well. it's simply what characters do in dramatic movie situations ;)

on the other hand, soul kitchen is set in a rather poor working-class milieu, where people tend to be less gentle of course...

Before watching it, I thought it was about a thriller type, but it turned out to be a very cool and great drama with great performance of the leads.

Yes, Germans yell a lot, especially in free time when they are getting drunk.
Plus, main characters are Greek, who are famous for yelling even more. :P

They are not all Germans. The main character, his brother and his friends are Greek and Greeks are famous for their temperament. The chef is a Gipsy, chiropractor and his family are Turkish... The boat maker ("captain", sailor like guy) is Greek (funny thing is that actor is Turkish, but he plays Greek character and those 2 nations don't get along very well in real life). Even the waitress is named Lucia, which is not a common German name, although her family name in the cast list (it was never mentioned in the movie) is.

Also, they are not yelling that much, the movie obviously didn't fit to you very well

They do really scream a lot in this movie.
And to answer your question. Nobody talks like that in Germany,
not the Germans, not the foreigners, neither the greek nor anybody.
It's just the style of the movie.