diff --git a/src/calibre/ebooks/mobi/debug/headers.py b/src/calibre/ebooks/mobi/debug/headers.py
index 6aaafbffa3..5b80a46f1b 100644
--- a/src/calibre/ebooks/mobi/debug/headers.py
+++ b/src/calibre/ebooks/mobi/debug/headers.py
@@ -163,6 +163,7 @@ class EXTHRecord(object):
                 501 : 'cdetype', # 4 chars (PDOC or EBOK)
                 502 : 'lastupdatetime',
                 503 : 'updatedtitle',
+                524 : 'language',
         }.get(self.type, repr(self.type))
 
         if (self.name in {'coveroffset', 'thumboffset', 'hasfakecover',
diff --git a/src/calibre/ebooks/mobi/reader/headers.py b/src/calibre/ebooks/mobi/reader/headers.py
index 51da05ea83..bfbffe546e 100644
--- a/src/calibre/ebooks/mobi/reader/headers.py
+++ b/src/calibre/ebooks/mobi/reader/headers.py
@@ -13,6 +13,7 @@ from calibre.utils.date import parse_date
 from calibre.ebooks.mobi import MobiError
 from calibre.ebooks.metadata import MetaInformation, check_isbn
 from calibre.ebooks.mobi.langcodes import main_language, sub_language, mobi2iana
+from calibre.utils.localization import canonicalize_lang
 
 NULL_INDEX = 0xffffffff
 
@@ -68,6 +69,14 @@ class EXTHHeader(object): # {{{
                     title = content.decode(codec)
                 except:
                     pass
+            elif idx == 524: # Lang code
+                try:
+                    lang = content.decode(codec)
+                    lang = canonicalize_lang(lang)
+                    if lang:
+                        self.mi.language = lang
+                except:
+                    pass
             #else:
             #    print 'unknown record', idx, repr(content)
         if title:
@@ -201,10 +210,11 @@ class BookHeader(object):
                     self.exth = EXTHHeader(raw[16 + self.length:], self.codec,
                             self.title)
                     self.exth.mi.uid = self.unique_id
-                    try:
-                        self.exth.mi.language = mobi2iana(langid, sublangid)
-                    except:
-                        self.log.exception('Unknown language code')
+                    if self.exth.mi.is_null('language'):
+                        try:
+                            self.exth.mi.language = mobi2iana(langid, sublangid)
+                        except:
+                            self.log.exception('Unknown language code')
                 except:
                     self.log.exception('Invalid EXTH header')
                     self.exth_flag = 0
diff --git a/src/calibre/ebooks/mobi/writer8/exth.py b/src/calibre/ebooks/mobi/writer8/exth.py
index daf6da62d6..72d1ce5a67 100644
--- a/src/calibre/ebooks/mobi/writer8/exth.py
+++ b/src/calibre/ebooks/mobi/writer8/exth.py
@@ -12,6 +12,7 @@ from struct import pack
 from io import BytesIO
 
 from calibre.ebooks.mobi.utils import utf8_text
+from calibre.utils.localization import lang_as_iso639_1
 
 EXTH_CODES = {
     'creator': 100,
@@ -35,6 +36,7 @@ EXTH_CODES = {
     'hasfakecover': 203,
     'lastupdatetime': 502,
     'title': 503,
+    'language': 524,
 }
 
 COLLAPSE_RE = re.compile(r'[ \t\r\n\v]+')
@@ -68,6 +70,10 @@ def build_exth(metadata, prefer_author_sort=False, is_periodical=False,
                     pass
                 else:
                     continue
+            if term == 'language':
+                d2 = lang_as_iso639_1(data)
+                if d2:
+                    data = d2
             data = utf8_text(data)
             exth.write(pack(b'>II', code, len(data) + 8))
             exth.write(data)