Agree with the others.
Staffing is usually very Germany centric (see reasoning below), but within Germany, everything can happen. As the German economy is very spread out across various economic centers (Munich, Hamburg, Cologne-Düsseldorf, Ruhrgebiet, Rhein-Main) there's actually a fair chance of having quite a few projects where you can sleep in your own bed (especially if you are Munich-based).
Regarding international staffing: The internal rates at which German consultants are charged to other country's offices on international projects tend to be quite high. So often times you may be prohibitively expensive to be staffed on a project run by an office in a different country. Much less the case for McK, but in general it is more likely that you have an international colleague on a German project than vice versa.
One more thing:
Think about your office choice wisely. Every office will have their own culture. (WARNING: Stereotypes ahead!)
- Berlin - lots of hipster beards
- Munich - posh, skiing and Oktoberfest are the two most important seasons of the year
- Hamburg - posh, but less in your face about it than Munich.
- Köln - probably the mellowest, go crazy around Carnival. People are quite open and approachable. A lot easier to make friends there if you're new in town than in Hamburg or Munich.
- Düsseldorf - quite similar to Köln, just a bit posher. Carnival is true there as well
- Frankfurt: Why would you go to Frankfurt? ;-)
As you will have guessed by now, there are pointed over-generalizations. However, each office does have their own flavor, which comes from the city it is based in (and obviously the people self-selecting to that office/city). Every office is proud of its city and probably considers it the best / coolest / nicest city in Germany. But it's usually just friendly banter. Except for Cologne and Düsseldorf, they're mortal enemies ;-)
Hope this helps,