When Kim and Jimmy are talking in the diner during the opening credits, she suggests using their powers for good instead of evil, whereupon Jimmy challenges her on how she can tell the difference. She says "It's like Potter Stewart said, we'll know it when we see it". Kim is referring to Supreme Court Justice Potter Stewart. The case of Jacobellis v Ohio (1964) was one in which the state was charging a theater owner with obscenity for showing Louis Malle's film "The Lovers". Justice Stewart famously wrote that he couldn't define obscenity but "I know it when I see it."
Wiedersehen means 'See you again' in German. The first mention was written in yellow spray paint on the rocky foundation which the demolition crew was trying to blast away.
The title can translate both as "(the) reunion" or as short and colloquial for "good-bye" (Auf Wiedersehen). This can be interpreted in both ways: as a hint at a "reunion" because Walter Ziegler intends to meet with his wife and because of the board's cynical comment that Jimmy, after having been rejected, could re-apply for reinstatement as attorney next year. Or as a hint at a "good-bye", because Walter Ziegler flees the premises, because he will probably face heavy punishment for going AWOL, because Jimmy and Kim are about to break up over their heavy fight and because Jimmy's career as an attorney might be over.