When adding books that have no language specified, do not automaticallly set the langauge to calibre's interface language. Fixes #830092 (Downloading and adding books causes default language to be assigned)

src/calibre/ebooks/metadata/opf2.py

@@ -1312,7 +1312,7 @@ class OPFCreator(Metadata):
             ncx_stream.flush()
 
 
-def metadata_to_opf(mi, as_string=True):
+def metadata_to_opf(mi, as_string=True, default_lang=None):
     from lxml import etree
     import textwrap
     from calibre.ebooks.oeb.base import OPF, DC
@@ -1328,7 +1328,8 @@ def metadata_to_opf(mi, as_string=True):
             '[http://calibre-ebook.com]'
 
     if not mi.languages:
-        lang = get_lang().replace('_', '-').partition('-')[0]
+        lang = (get_lang().replace('_', '-').partition('-')[0] if default_lang
+                is None else default_lang)
         mi.languages = [lang]
 
     root = etree.fromstring(textwrap.dedent(

src/calibre/ebooks/metadata/worker.py

@@ -33,7 +33,7 @@ def serialize_metadata_for(formats, tdir, id_):
     if not mi.application_id:
         mi.application_id = '__calibre_dummy__'
     with open(os.path.join(tdir, '%s.opf'%id_), 'wb') as f:
-        f.write(metadata_to_opf(mi))
+        f.write(metadata_to_opf(mi, default_lang='und'))
     if cdata:
         with open(os.path.join(tdir, str(id_)), 'wb') as f:
             f.write(cdata)