Adding data parameter for Grammarly API to support (german) language

Hello guys
I am trying to get my grammarly-LSP in AstroVim to work with German language and preferably both German and English. Could someone please point me to the file which handles the request and tell me, where I could add the language=de_DE,en_US - if possible - to the POST request. I think I have looked into every file in ~/.local/share/nvim/* and all other files I could find. Am I just blind as always or is this indeed a tricky one?

Thanks