Are there precedence of legal requirements for diacritic support? Does it extend to all latin diacritics or popular regional subsets. I remember a Chinese schol

Are there precedence of legal requirements for diacritic support? Does it extend to all latin diacritics or popular regional subsets. I remember a Chinese scholar pushing for multiple language email address standard years ago, thinking it would be neat (and profoundly inpractical). Also maybe I'm misremembering, I swear I've seen arabic email addresses before.