http://www.dartmouth.edu/~german/Grammatik/Nouns/accusative.html (bottom of the page)
Don't take this one too serious.
That doesn't stop the "Rolling Stones" from singing:
"A smile relieves a heart that grieves, remember what I said. I'm not waiting on a lady, I'm just waiting on a friend. I'm just waiting on a friend, just waiting on a friend. I'm just waiting on a friend, I'm just waiting on a friend, just waiting on a friend."
It really depends on where you are, at least in the US I think it is regional. For me (grew up in the Chicago area) I use "wait on" in both cases for "wait on - being served" and "wait for someone/something". But to my wife who grew up 2.5 hours south of me "wait on" is for being served only.
"Wir warten auf ihn." has only two possible English translations no matter what the context will be.
We are waiting for him.
We are waiting on him. (second version more likely used in: We are waiting on a friend.)
If watching for is given as an option, -it is wrong!