@shikis/rehype doesn't use the `fallbackLanguage` if `lazy` is used.

--- a/dist/core.mjs
+++ b/dist/core.mjs
@@ -1,6 +1,8 @@
 import { visit } from 'unist-util-visit';
 import { toString } from 'hast-util-to-string';
 
+import { bundledLanguages } from 'shiki';
+
 const InlineCodeHandlers = {
   "tailing-curly-colon": (_tree, node) => {
     const raw = toString(node);
@@ -90,8 +92,12 @@ function rehypeShikiFromHighlighter(highlighter, options) {
     const languageQueue = [];
     const queue = [];
     function getLanguage(lang) {
       if (!lang)
         return defaultLanguage;
+
+      if (!(lang in bundledLanguages))
+        return fallbackLanguage;
+
       if (highlighter.getLoadedLanguages().includes(lang))
         return lang;
       if (lazy) {