There are many different languages in Teyvat, most of them based on real world languages.
Summary
Common language
It is the written language used by all the nations of Teyvat to communicate with each other, just like English in the real world. The letters are apparently inspired by Latin letters reflected backwards, although not all. Apparently, unlike English, there are no capital letters or punctuation.
Lengua Hillichurl
Hilichurls also have a language, Hilichurlio. Not much is known about it, except for "Olah," which means "Hello," and some fragments of poetry of questionable value.
Lengua originaria de Liyue
The written language used in Liyue, first seen during the Ningguang scene at the end of Act II, Chapter I. The characters are probably based on Chinese characters from the real world or at least from some eastern nation, although there is still no information about what each character means or if there is any way to associate the characters with actual Chinese characters.
Unknown language
This language can be found in domains or chests that are locked by a red circle. Like common written language, it is based on Latin letters, but unlike common language, the words and grammar are actually based on classical Latin. Although there are no capital letters, there is punctuation.
The unknown language can also be seen in the Teyvat chapter story preview, which is used to write Latin phrases for the theme of each nation's history. Since Dainsleif is the narrator and is from the nation of Khaenri'ah, the unknown language may be the written language of Khaenri'ah.
