fix(backend): fill lang when importing mst (#1929)

Closes #1928

Co-authored-by: wangzhao <wangzhao@megvii.com>

backend/import.py

@@ -92,6 +92,7 @@ def import_tree(
             state=state,
             origin=self.origin,
             active=active,
+            lang=root_msg.lang or "en",
         )
         self.db.add(mts)
         return mts, root_msg