Fix plural unit detection for some formats

Fixes rb#577
Fixes rb#578
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/unit.py

@@ -499,7 +499,7 @@ class Unit(models.Model, LoggerMixin):
         """
         Checks whether message is plural.
         """
-        return is_plural(self.source)
+        return is_plural(self.source) or is_plural(self.target)
 
     def get_source_plurals(self):
         """