I have yiffi.net blocked (the whole instance) and yet that shit still shows up on the bottom right of my page under "random posts." Any idea how to stop it from appearing there as well?

Almost exactly what I would have written!

I wish you didn't have to see a magazine you don't want to see to block it

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();
})();

Thanks, I didn't know!

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