I would not translate Alaaf as hurray. But I can understand why it would be.
I would consider Alaaf to be untranslateable much like the corresponding Helau and Dollau. All three are calls that are only heard during the carnival season.
Alaaf is a carnival call typical to Cologne (Kölle Alaaf!) and Bonn, while Helau is used primarily in Düsseldorf and Mainz. (Dollau was introduced to me as a smaller, more localised variant of Helau in the Rhine-Hessia region.) These calls would be used:
- When the carnival parade is passing
- During the so-called Sitzungen (conferences would be a very bad translation; it’s more like a comedians’ group show but that doesn’t really capture it either) at the end of somebody’s act.
Also, some people use it as a greeting during carnival season to express how much they themselves are carnivalistic. There is a lot more to these calls, (when they are used, how they are used, what to do while calling them) that might just be too much for this platform. In any case, not even all Germans know everything about these carnival traditions; the farther you get away from the South and West, the less they do.
A proper translation for hurray is Hurra.