Kbin does have tags for the language of posts, and they are accurately represented in these German posts, but I can't find a way to filter them without just blocking the server!

As a quick fix, a user script:

// ==UserScript==
// @name         Kbin: delete articles in other languages
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Auto-delete posts in languages you do not speak.
// @author       luphoria (https://kbin.social/u/trent)
// @match        https://kbin.social/
// @icon         https://www.google.com/s2/favicons?sz=64&domain=kbin.social
// @grant        none
// ==/UserScript==

(function() {
  'use strict';
  const allowedLangs = ["en", "es"]; // Edit this to all of the languages you can speak / want to see in your feed

  let deleteUnwantedPosts = () => {
    const postTags = document.getElementsByClassName("kbin-bg");

    for (let i = 0; i < postTags.length; i++) {
      let postTag = postTags[i];
      if (postTag && postTag.textContent && postTag.textContent !== "OC" && !allowedLangs.includes(postTag.textContent)) { // OC tags are the only elements (i know of) with the same class.
        // Delete element's parent's parent's parent
        if (postTag.parentElement.children[0].textContent) {
          console.log(`Removing post labeled \`${postTag.textContent}\`: "${postTag.parentElement.children[0].textContent}"`);
          postTag.parentElement.parentElement.parentElement.remove();
        }
      }
    }
  }
  deleteUnwantedPosts(); // on load

  // for continuous feeds
  let observeDOMChanges = () => {
    const observer = new MutationObserver(function(mutationsList) {
      deleteUnwantedPosts();
    });
    observer.observe(document.body, { childList: true, subtree: true });
  }
  observeDOMChanges();
})();

I think lemmy has language in account settings which presumably filters content based on it, but kbin doesn't (yet).