Fix #802

src/calibre/ebooks/metadata/__init__.py

@@ -38,7 +38,7 @@ def copy(mi):
                 setattr(ans, attr, getattr(mi, attr))
         
     
-    def __init__(self, title, authors=['Unknown']):
+    def __init__(self, title, authors=[_('Unknown')]):
         '''
         @param title: title or "Unknown" or a MetaInformation object
         @param authors: List of strings or []

src/calibre/ebooks/metadata/opf.py

@@ -511,6 +511,8 @@ def __init__(self, base_path, *args, **kwargs):
                 path = path[len(self.base_path)+1:]
                 manifest.append((path, mt))
             self.manifest = manifest
+        if not self.authors:
+            self.authors = [_('Unknown')]
     
     def create_manifest(self, entries):
         '''

src/calibre/library/database.py

@@ -1427,6 +1427,8 @@ def export_to_dir(self, dir, indices, byauthor=False, single_dir=False,
                     open(cpath, 'wb').write(cover)
                     mi.cover = cname
                 f = open(os.path.join(base, sanitize_file_name(name)+'.opf'), 'wb')
+                if not mi.authors:
+                    mi.authors = [_('Unknown')]
                 mi.render(f)
                 f.close()