A few Germany-specific suggestions, since it's the only country I can claim to know
Don't ask random people personal questions about the Nazis or the Third Reich.
Behave respectfully while visiting memorials and concentration camp museums. To you, it might be a fascinating experience, or a story from far away and long ago, but many Germans still suffer deeply because of their past.
On a lighter note: Going out to dinner might take much more time than in the US. Waiting half an hour or more for the food to arrive is quite usual. On the other hand there is no need to tip exactly 20% or more. Waitstaff get halfway decent hourly wages.
More than I make in any case.
And if you're from an English-speaking country: Many people speak at least a little English and just LOVE to put it into practise. If you just grin and bear it, you'd make many of us very happy. Including me.