I don't know about Japan (but I imagine you are wrong), but Germany is not an "occupied country" any more. One of the side-effects of reunification was a peace treaty formally ending that status. With the USSR giving up control of "their part" the western allies had no reason not to do the same. Margaret Thatcher was less than enthusiastic about this but could do little once George Bush the Elder (I think it was him and not Clinton) had made the decision. Thatcher was dumped around then anyway and John Major was a lot less bigoted.
Going back to the actual story here: At the time the Nazis took power, a large proportion of German scientists were jewish. Once their initial fears proved to be overly optimistic, those who could headed for the exits and were mostly not replaced. At that point German as the "Language of Science" was pretty much dead, losing WW2 just confirmed this.