[5.1.x] Fixed #35876 -- Displayed non-ASCII fieldset names when rendering ModelAdmin.fieldsets.

Thank you to Namhong Kim for the report, and to Mariusz Felisiak and Marijke Luttekes for the review.

Regression in 01ed59f753.

Backport of 2c029c718f from main.

tests/admin_views/admin.py

@@ -237,6 +237,7 @@ class ArticleAdmin(ArticleAdminWithExtraUrl):
             "Some other fields",
             {"classes": ("wide",), "fields": ("date", "section", "sub_section")},
         ),
+        ("이름", {"fields": ("another_section",)}),
     )
 
     # These orderings aren't particularly useful but show that expressions can

tests/admin_views/tests.py

@@ -2508,6 +2508,19 @@ class AdminViewPermissionsTest(TestCase):
         self.assertContains(
             response, '<input type="submit" value="Save and view" name="_continue">'
         )
+        self.assertContains(
+            response,
+            '<h2 id="fieldset-0-0-heading" class="fieldset-heading">Some fields</h2>',
+        )
+        self.assertContains(
+            response,
+            '<h2 id="fieldset-0-1-heading" class="fieldset-heading">'
+            "Some other fields</h2>",
+        )
+        self.assertContains(
+            response,
+            '<h2 id="fieldset-0-2-heading" class="fieldset-heading">이름</h2>',
+        )
         post = self.client.post(
             reverse("admin:admin_views_article_add"), add_dict, follow=False
         )