Now, I know as much as German morphology as John McCain* did about economics. But this didn't look right:
It was taken in the Wisconsin Union's famous Rathskeller, as the sign suggests. (We've talked about the 'Rath' before, as a search of this blog will show.) I've talked about this to people who know some German and they say that Keller is masculine, so the nominative form is der Rathskeller. In parallel constructions in German, you get dative case, like "Willkommen in der Heimat" 'welcome home' or "Willkommen im Klub" 'welcome to the club', which would give "Willkommen im Rathskeller" here, presumably. In fact, Google gives lots of examples with im if you use the modern spelling Ratskeller. At any rate, a nominative determiner after a preposition has got to be off and I was ready to jump on this as a blunder.
But some folks around here who know German also know something about language contact and suggested alternatives. For instance, you don't have to read this as German, but can treat it as an English sentence with German loanwords: willkommen is well-established here on signs and such, and Der Rathskeller is the real name of the place. The problem with that is that I can't get the German-like preposition in to work: Willkommen to Der Rathskeller would be the outcome. And if that's a Wisconsin-English-ism, I haven't heard it.
Or maybe somebody insisted on treating Der Rathskeller as an invariant phrase, so that it's German with a switch to English for the last noun phrase, which is of course borrowed from German. The cap in Der would be consistent with that — distinctly not German-looking. Very odd to a German eye, I'll bet, but in line with the Union's consistent use of the determiner with that noun.
*John McCain was once a candidate for president of the United States. He ran on the 'Republican' ticket. I'm sure he has a wikipedia entry.