-                    tag, created = Tag.objects.get_or_create(
-                                        slug=slughifi(theme_name),
-                                        category='theme')
-                    if created:
-                        tag.name = theme_name
-                        tag.sort_key = sortify(theme_name.lower())
-                        tag.save()
-                    themes.append(tag)
+                    if lang == settings.LANGUAGE_CODE:
+                        # Allow creating themes if book in default language.
+                        tag, created = Tag.objects.get_or_create(
+                                            slug=slughifi(theme_name),
+                                            category='theme')
+                        if created:
+                            tag.name = theme_name
+                            setattr(tag, "name_%s" % lang, theme_name)
+                            tag.sort_key = sortify(theme_name.lower())
+                            tag.save()
+                        themes.append(tag)
+                    elif lang is not None:
+                        # Don't create unknown themes in non-default languages.
+                        try:
+                            tag = Tag.objects.get(category='theme',
+                                    **{"name_%s" % lang: theme_name})
+                        except Tag.DoesNotExist:
+                            pass
+                        else:
+                            themes.append(tag)