Can a soldier take leave and fly overseas to a base in Germany or anywhere?

As long as your leave is approved, then yes. As SFC Robinson mentions, depending on where you go you may need to have a security briefing, or you may be barred from going there. Just as with traveling outside of a certain radius, you will have some additional things to take into consideration such as supplying how you get there (travel plans, flight itinerary, etc) and whatever else your CoC will require of you for foreign travel. Depending on mission requirements, you may not be allowed to go beyond a certain recall radius.

Germany, yes. Anywhere, no. Get with your S-2 for udpated list of places that are restricted/of-limits to service members. Enjoy your leave!