fix solution str representation

apps/participation/models.py

@@ -561,7 +561,7 @@ class Solution(models.Model):
     def __str__(self):
         return _("Solution of team {team} for problem {problem}")\
             .format(team=self.participation.team.name, problem=self.problem)\
-            + (" " + _("for final") if self.final_solution else "")
+            + (" " + str(_("for final")) if self.final_solution else "")
 
     class Meta:
         verbose_name = _("solution")