fix #2555 language x in preview

evap/staff/tests/test_views.py

@@ -2465,11 +2465,18 @@ def setUpTestData(cls):
         cls.test_users = [cls.manager]
         cls.url = reverse("staff:evaluation_preview", args=[cls.evaluation.pk])
 
+        cls.evaluation_un = baker.make(Evaluation, main_language=Evaluation.UNDECIDED_MAIN_LANGUAGE)
+        cls.url2 = reverse("staff:evaluation_preview", args=[cls.evaluation_un.pk])
+        cls.evaluation_un.general_contribution.questionnaires.set([baker.make(Questionnaire)])
+
     def test_without_questionnaires_assigned(self):
         # regression test for #1747
         self.evaluation.general_contribution.questionnaires.set([])
         self.app.get(self.url, user=self.manager, status=200)
 
+    def test_lang_undecided(self):
+        self.app.get(self.url2, user=self.manager, status=200)
+
 
 class TestEvaluationImportPersonsView(WebTestStaffMode):
     @classmethod

evap/student/views.py

@@ -252,7 +252,11 @@ def render_vote_page(
     dropout: bool,
     for_rendering_in_modal: bool = False,
 ) -> HttpResponse:
-    language = request.GET.get("language", evaluation.main_language)
+    fallback_language = (
+        evaluation.main_language if evaluation.main_language != evaluation.UNDECIDED_MAIN_LANGUAGE else "en"
+    )
+    language = request.GET.get("language", fallback_language)
+
     with translation.override(language):
         form_groups = get_vote_page_form_groups(request, evaluation, preview=preview, preselect_no_answer=dropout)