What language is the Koii node written in?

The Koii node is written natively in typescript, but may undergo a rebuild in the next year or two if performance improvements become more important. Typescript currently meets our needs and gives the Koii team an extra edge due to the number of compatible libraries and existing standards.

English language

As for the official answer by MetaBrynn, that is true, but if were talking about language/languages, it is written on English language. I think in the future versions of the node, there will be language switcher.