[5.1.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.

docs/releases/4.2.15.txt

@@ -0,0 +1,14 @@
+===========================
+Django 4.2.15 release notes
+===========================
+
+*Expected August 6, 2024*
+
+Django 4.2.15 fixes a regression in 4.2.14.
+
+Bugfixes
+========
+
+* Fixed a regression in Django 4.2.14 that caused a crash in
+  ``LocaleMiddleware`` when processing a language code over 500 characters
+  (:ticket:`35627`).

docs/releases/5.0.8.txt

@@ -19,3 +19,7 @@ Bugfixes
 * Fixed a crash when creating a model with a ``Field.db_default`` and a
   ``Meta.constraints`` constraint composed of ``__endswith``, ``__startswith``,
   or ``__contains`` lookups (:ticket:`35625`).
+
+* Fixed a regression in Django 5.0.7 that caused a crash in
+  ``LocaleMiddleware`` when processing a language code over 500 characters
+  (:ticket:`35627`).

docs/releases/index.txt

@@ -48,6 +48,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   4.2.15
    4.2.14
    4.2.13
    4.2.12