Luckily I found the problem. If I use the files of "The Vox Populi Translation Project" and play the game in German. I had this issue:
Wissenschaftlers: +2

Kultur.
I know that everybody that don't speak German can not see the issue here. In English the text looks like this:
Scientists: +2

Culture
The problem is this text key that is responsible for the text: <Text>{1_SpecialistName:textkey}
s: +{3_Num} {4_IconString} {2_YieldType:textkey}</Text>
The game takes the name of the specialist and a "s" will be added to it. If I play the game in English, this makes sense, so In English all scientists (plural) generate +2

Culture.
In German however, the word for scientist ist Wissenschaftler (singular) and the same word counts for plural: Wissenschaftler.
So by playing the game in German, the text key needs to be altered, so that the game will not add a "s" to the specialist:
<Text>{1_SpecialistName:textkey}: +{3_Num} {4_IconString} {2_YieldType:textkey}</Text>
Then the text will be correct: Wissenschaftler: +2

Kultur.
I wonder if it is possible to add anything to the text key, so that the game will take the plural for of a specialist automatically.