diff --git a/src/calibre/devices/apple/driver.py b/src/calibre/devices/apple/driver.py
index 845423247c..59bef5b334 100644
--- a/src/calibre/devices/apple/driver.py
+++ b/src/calibre/devices/apple/driver.py
@@ -2347,7 +2347,7 @@ class ITUNES(DriverBase):
                         self.log.info("   add timestamp: %s" % metadata.timestamp)
 
                 # Force the language declaration for iBooks 1.1
-                metadata.language = get_lang()
+                metadata.language = get_lang().replace('_', '-')
                 if DEBUG:
                     self.log.info("   rewriting language: <dc:language>%s</dc:language>" % metadata.language)
 
diff --git a/src/calibre/ebooks/chm/input.py b/src/calibre/ebooks/chm/input.py
index a3a49296d2..820178408c 100644
--- a/src/calibre/ebooks/chm/input.py
+++ b/src/calibre/ebooks/chm/input.py
@@ -92,7 +92,7 @@ class CHMInput(InputFormatPlugin):
             metadata.add('identifier', mi.isbn, attrib={'scheme':'ISBN'})
         if not metadata.language:
             oeb.logger.warn(u'Language not specified')
-            metadata.add('language', get_lang())
+            metadata.add('language', get_lang().replace('_', '-'))
         if not metadata.creator:
             oeb.logger.warn('Creator not specified')
             metadata.add('creator', _('Unknown'))
diff --git a/src/calibre/ebooks/html/input.py b/src/calibre/ebooks/html/input.py
index 73fd020d7b..73bc22be66 100644
--- a/src/calibre/ebooks/html/input.py
+++ b/src/calibre/ebooks/html/input.py
@@ -329,7 +329,7 @@ class HTMLInput(InputFormatPlugin):
             metadata.add('identifier', mi.isbn, attrib={'scheme':'ISBN'})
         if not metadata.language:
             oeb.logger.warn(u'Language not specified')
-            metadata.add('language', get_lang())
+            metadata.add('language', get_lang().replace('_', '-'))
         if not metadata.creator:
             oeb.logger.warn('Creator not specified')
             metadata.add('creator', self.oeb.translate(__('Unknown')))
diff --git a/src/calibre/ebooks/metadata/opf2.py b/src/calibre/ebooks/metadata/opf2.py
index 579398d3b0..0bb0c570ed 100644
--- a/src/calibre/ebooks/metadata/opf2.py
+++ b/src/calibre/ebooks/metadata/opf2.py
@@ -1069,7 +1069,8 @@ class OPFCreator(MetaInformation):
             dc_attrs={'id':__appname__+'_id'}))
         if getattr(self, 'pubdate', None) is not None:
             a(DC_ELEM('date', self.pubdate.isoformat()))
-        a(DC_ELEM('language', self.language if self.language else get_lang()))
+        a(DC_ELEM('language', self.language if self.language else
+            get_lang().replace('_', '-')))
         if self.comments:
             a(DC_ELEM('description', self.comments))
         if self.publisher:
@@ -1194,7 +1195,8 @@ def metadata_to_opf(mi, as_string=True):
         factory(DC('identifier'), mi.isbn, scheme='ISBN')
     if mi.rights:
         factory(DC('rights'), mi.rights)
-    factory(DC('language'), mi.language if mi.language and mi.language.lower() != 'und' else get_lang())
+    factory(DC('language'), mi.language if mi.language and mi.language.lower()
+            != 'und' else get_lang().replace('_', '-'))
     if mi.tags:
         for tag in mi.tags:
             factory(DC('subject'), tag)
diff --git a/src/calibre/ebooks/oeb/reader.py b/src/calibre/ebooks/oeb/reader.py
index ebe6e78d08..d7d7bbf725 100644
--- a/src/calibre/ebooks/oeb/reader.py
+++ b/src/calibre/ebooks/oeb/reader.py
@@ -131,7 +131,7 @@ class OEBReader(object):
         stream = cStringIO.StringIO(etree.tostring(opf))
         mi = MetaInformation(OPF(stream))
         if not mi.language:
-            mi.language = get_lang()
+            mi.language = get_lang().replace('_', '-')
         self.oeb.metadata.add('language', mi.language)
         if not mi.title:
             mi.title = self.oeb.translate(__('Unknown'))