For src/content/docs/de/about/book.md adding to locales.json

Conflikt solved in locales.json with ko and de

Author: Chris2000SP

locales.json

@@ -22,5 +22,9 @@
   "ko": {
     "label": "한국어",
     "lang": "ko"
+  },
+  "de": {
+    "label": "German",
+    "lang": "de"
   }
 }