Store languages in a set

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/machine/base.py

@@ -198,7 +198,7 @@ class MachineTranslation(object):
 
         # Download
         try:
-            languages = self.download_languages()
+            languages = set(self.download_languages())
         except Exception as exc:
             self.report_error(
                 exc,