[5.0.x] Fixed #35627 -- Raised a LookupError rather than an unhandled ValueError in get_supported_language_variant().

LocaleMiddleware didn't handle the ValueError raised by
get_supported_language_variant() when language codes were
over 500 characters.

Regression in 9e9792228a.

Backport of 0e94f292cd from main.

tests/i18n/tests.py

@@ -1673,14 +1673,13 @@ class MiscTests(SimpleTestCase):
             g("xyz")
         with self.assertRaises(LookupError):
             g("xy-zz")
-        msg = "'lang_code' exceeds the maximum accepted length"
         with self.assertRaises(LookupError):
             g("x" * LANGUAGE_CODE_MAX_LENGTH)
-        with self.assertRaisesMessage(ValueError, msg):
+        with self.assertRaises(LookupError):
             g("x" * (LANGUAGE_CODE_MAX_LENGTH + 1))
         # 167 * 3 = 501 which is LANGUAGE_CODE_MAX_LENGTH + 1.
         self.assertEqual(g("en-" * 167), "en")
-        with self.assertRaisesMessage(ValueError, msg):
+        with self.assertRaises(LookupError):
             g("en-" * 167, strict=True)
         self.assertEqual(g("en-" * 30000), "en")  # catastrophic test
 
@@ -1734,6 +1733,7 @@ class MiscTests(SimpleTestCase):
             ("/i-mingo/", "i-mingo"),
             ("/kl-tunumiit/", "kl-tunumiit"),
             ("/nan-hani-tw/", "nan-hani-tw"),
+            (f"/{'a' * 501}/", None),
         ]
         for path, language in tests:
             with self.subTest(path=path):
@@ -2012,6 +2012,11 @@ class CountrySpecificLanguageTests(SimpleTestCase):
         lang = get_language_from_request(request)
         self.assertEqual("bg", lang)
 
+    def test_get_language_from_request_code_too_long(self):
+        request = self.rf.get("/", headers={"accept-language": "a" * 501})
+        lang = get_language_from_request(request)
+        self.assertEqual("en-us", lang)
+
     def test_get_language_from_request_null(self):
         lang = trans_null.get_language_from_request(None)
         self.assertEqual(lang, "en")