In some languages (Spanish, for example) it’s double v.

It’s a long story. In short: In Latin script u and v were the same letter “u” but had two pronunciations depending on whether it was being used as a vowel or consonant. But when adapting the alphabet to Germanic languages (including Old English) the same two sounds were from two different letters, so they put two "u"s together to make double u: vv.

The full story: https://www.youtube.com/watch?v=sg2j7mZ9-2Y

U and V used to be the same letter

In Swedish I pronounce y as y. It has its own sound and doesn’t sound like another letter, so it can’t be written as a combination of other letters.

Üpsilon

In Flanders (at least where I’m at) we usually say I grec, but when doing math or reciting the alphabet, we say IJ.

Usually same as our compound letter “ij”, similar but not quite how you’d prononuce the word “eye”. Less commonly it’s pronounced as “i-grec” (greek i) or “ypsilon”.

Sound like igrek.