For instance é in Dutch is capitalized as E, but in French it is É

Sure, but we’re just talking about string comparison rules, and Unicode sees all three of those as being equal. For example, a search engine that uses proper case folding rules in its indexer should return results for “entrée” if you search for “entree”, “Čech” if you search for “cech”, etc.

It should imo only be used for fuzzy searching doubles, which you probably will do with ToUpper

You can’t just use ToUpper for comparisons due to issues like you mentioned, and the Turkish i problem. You need to do proper case-insensitive comparisons, which is where the Unicode case folding rules are used.