The written language usually uses plain English with a custom font on top that replaces Latin characters with custom glyphs. The font is called Caro Invitica and was made by Tyfin.
Text from chat messages, sign and anvil item renaming are first transformed by transliterating all non-Latin characters into Latin characters.
Manual Usage
The font can be manually applied to anything that uses Chat/Text Components by setting the font
filed to biomancy:caro_invitica.
Vanilla Commands:
/give @p stick{display:{Name:"[{\"text\":\"Hello World\",\"font\":\"biomancy:caro_invitica\"}]"}}/title @a title {"text":"Hello World","font":"biomancy:caro_invitica"}
Bookshelf Commands:
/bookshelf font say biomancy:caro_invitica/bookshelf font item biomancy:caro_invitica/bookshelf font book biomancy:caro_invitica/bookshelf font block biomancy:caro_invitica