How to query language with KQL?

Heyo. This is my function:

  const info = await $fetch(api, {
    method: "post",
    body: {
      query: "site",
      language: "de",
      select: {
        info: {
          query: "",
          language: "de",
        contact: "",

But it’s still the en version. I checked on my backend and the de.txt and en.txt files exist.
How can I query the de versions?

I think you have to set the x-language header.