Which collation should I choose for a muiti-language website?
Generally speaking, one of the Unicode variants is probably the best for broad language support - UTF-8 is going to use less memory per codepoint, and thus will have a slight advantage in any time/space tradeoffs you find yourself in need of making; however, I think there are some of the more esoteric languages/scripts that UTF-8 cannot represent (but I'm not 100% certain of that, I haven't done an exhaustive study on the matter).

This Wikipedia article might be enlightening on the dis/advantages of each.