How well does Wikipedia deal with the history of mathematics?

Short answer: not too well. There is a lot of fictional and irrelevant material, and far too little mathematics.

How does the English Wikipedia compare to the German one?

Short answer: the German one is somewhat more reliable on facts, and also often better in how it deals with mathematical exposition. It is also less suitable for non-mathematicians (as far as I can tell). Both can be improved substantially.

How can both do better?

Oracular answer: understand what the problems are, then solve them.