That's definitely a reasonable thing for governments to care about, since governments generally need to keep track of people's names in some way or other. It's

That's definitely a reasonable thing for governments to care about, since governments generally need to keep track of people's names in some way or other. It's probably a good idea if government computers can keep track of the name, so a minimum requirement would probably be "can be represented by unicode glyphs". So attempts like Prince's should probably be disqualified. And even in the unicode set there are characters that may be problematic - Record Separators, Zero Width Space, Pile of Poo emoji springs to mind as examples, even if the later one might be doable. It's hard to address a letter to a person whose name just consists of a mix of different whitespace characters (especially when their neighbor is a different mix whitespace characters). So, yes, government probably should care about this, at least a little bit. That said, Æ should probably be allowed. It's a standard character in several living languages.