There is a city in America whose population is mostly descended from Germans.

This has nothing to do with the English name of Germany. If Germany is Deutschland in German, do you still need to find a city starting with D?

I haven't heard of the specific city. But most of the residents in Wisconsin are German descendants of immigrants in the mid-Kloc-0/9th century, so most cities in Wisconsin belong to your problem.

If it has anything to do with beer, it may be Milwaukee, which is regarded as the most taciturn German city in America.