add route for an argument detail in a given language

picampaign/campaign/views.py

@@ -38,3 +38,8 @@ class ArgumentaryViewSet(viewsets.ReadOnlyModelViewSet):
         argumentaries = self.queryset.filter(campaign=campaign_pk)
         serializer = self.serializer_class(argumentaries.all(), many=True)
         return Response(serializer.data)
+
+    def retrieve(self, request, pk=None, campaign_pk=None):
+        argument = self.queryset.get(lang=pk, campaign=campaign_pk)
+        serializer = self.serializer_class(argument)
+        return Response(serializer.data)