I'm only talking about ASCII letters. Umlauts are not in the basic character set anyway, so we can't provide any universal guarantee for those.

For ASCII letters (with the same case), we can guarantee ordering. I think we can even guarantee that lowercase and uppercase letters are not interleaved, but in two disjoint ranges, and even guarantee that there is a constant offset between lowercase and uppercase letters. The only thing we cannot guarantee is full contiguity.