diff --git a/src/calibre/ebooks/metadata/mobi.py b/src/calibre/ebooks/metadata/mobi.py
index 5863f00d6a..ab475f33a8 100644
--- a/src/calibre/ebooks/metadata/mobi.py
+++ b/src/calibre/ebooks/metadata/mobi.py
@@ -18,6 +18,7 @@ from calibre.ebooks.mobi import MobiError, MAX_THUMB_DIMEN
 from calibre.ebooks.mobi.utils import rescale_image
 from calibre.ebooks.mobi.langcodes import iana2mobi
 from calibre.utils.date import now as nowf
+from calibre.utils.localization import canonicalize_lang, lang_as_iso639_1
 
 def is_image(ss):
     if ss is None:
@@ -401,6 +402,13 @@ class MetadataUpdater(object):
         if getattr(mi, 'book_producer', False):
             update_exth_record((108, mi.book_producer.encode(self.codec, 'replace')))
 
+        # Set langcode in EXTH header
+        if not mi.is_null('language'):
+            lang = canonicalize_lang(mi.language)
+            lang = lang_as_iso639_1(lang) or lang
+            if lang:
+                update_exth_record((524, lang.encode(self.codec, 'replace')))
+
         # Include remaining original EXTH fields
         for id in sorted(self.original_exth_records):
             recs.append((id, self.original_exth_records[id]))