"If lets say a Turkish citizen works here since 20 years, she usually has more understanding of the issues of local politics here (which street to close, new busline or not, where do we need a playground) than in any Turkish town she still may have to right to vote for the local elections. The main idea just seems sensible to me, just the concrete conditions open to debate."

But why in heaven's name would anyone live in Germany "temporarily" 20 years? Why not just admit you've migrated and become a citizen? Is this something special to Germany?

In the US immigrants are encouraged to gain citizenship, it's a great priviledge. If someone who had lived in this country for 20 years wanted to vote, I'd drive them to the post office and help them fill out the citizenship forms.

"Just a question: If someone from lets say Hawaii moves to Maine, how long does she have to wait before being allowed to vote in Maine? And is there a test whether she has enough knowledge of the local issues?"

No test, usually just the requirement that they live in the state a minimal period of time which varies from state to state and that they register to vote. The main problem is convincing someone who moves to another state to register since then they'll be expected to re-register their car which can be quite expensive, particularly if you move into California.

There is a huge difference, though. The people I'm talking about are alreadly US citizens. The differences between the various states were part of the impetous to abandon the Articles of Confederation and adopt a more uniform Constitution.