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

--- a/dist/core.mjs
+++ b/dist/core.mjs
@@ -2,6 +2,8 @@ import { isSpecialLang } from 'shiki/core';
 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);
@@ -95,6 +97,8 @@ function rehypeShikiFromHighlighter(highlighter, options) {
         return defaultLanguage;
       if (highlighter.getLoadedLanguages().includes(lang) || isSpecialLang(lang))
         return lang;
+      if (!(lang in bundledLanguages))
+        return fallbackLanguage;
       if (lazy) {
         languageQueue.push(lang);
         return lang;