As an American in Germany, people’s first reaction to me is usually one of the following:
1) Oh cool! You’re American. Want to hang out and help me practice my English?
2) Great, another loud Ami...
3) Wait, why do you not have a thick American accent when you speak German?
I wish there was more of a variety in initial reactions, but those are usually what I get. There is nothing wrong with any of those reactions, but it took some major adjusting to get used to people talking to me only because they know I am American. I usually speak German with my German friends, and English with my native English speakers, so I have had people approach me with my friends because they realize we are American. It is fairly common.
Germans learn English for years in school and love to practice with native speakers. It is rather sweet at times. Sometimes though, it is weird because completely random people will come up to you because they hear your accent. It happened to me twice on Halloween. I was with some Australian and American friends, so we were speaking English to each other. A young woman actually started a conversation with me in the bathroom while we were washing our hands because she had heard me say something to my friend before walking in. She was very nice. I just cannot help but remember the situation as a bit absurd.
