TranslationCatalog.__getitem__ gives wrong priority to non-plural translations when plural forms mismatch

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Description

When a third-party package (e.g. django-allauth) has a slightly different Plural-Forms header than the project's own translations (e.g. plural=n != 1 vs plural=(n != 1)), TranslationCatalog.update() correctly prepends a separate catalog to preserve plural lookup correctness.

However, TranslationCatalog.__getitem__() iterates catalogs in order and returns the first match. This means the prepended catalog wins for all lookups, including non-plural gettext() calls where plural form separation is irrelevant. This silently overrides higher-priority translations (from LOCALE_PATHS or later INSTALLED_APPS) with lower-priority ones.

Steps to reproduce

1. Create a Django project with a translation in LOCALE_PATHS:

   # project/locale/nl/LC_MESSAGES/django.po
   # Plural-Forms: nplurals=2; plural=(n != 1);
   msgid "Activate"
   msgstr "Activeren"
   

2. Install a third-party app (e.g. django-allauth) that defines the same msgid with a different Plural-Forms header:

   # allauth/locale/nl/LC_MESSAGES/django.po  
   # Plural-Forms: nplurals=2; plural=n != 1;
   msgid "Activate"
   msgstr "Activeer"
   

Note: n != 1 and (n != 1) are functionally identical and compile to identical bytecode, but Python's gettext.c2py() produces functions with different __code__ objects. Django's TranslationCatalog.update() compares __code__ to decide whether to merge or prepend.

3. gettext("Activate") returns "Activeer" (allauth's version) instead of "Activeren" (the project's version), even though LOCALE_PATHS should have highest priority per Django's documentation.

Root cause

In django/utils/translation/trans_real.py:

class TranslationCatalog:
    def update(self, trans):
        # Mismatched plural -> prepend to position 0
        for cat, plural in zip(self._catalogs, self._plurals):
            if trans.plural.__code__ == plural.__code__:
                cat.update(trans._catalog)
                break
        else:
            self._catalogs.insert(0, trans._catalog.copy())
            self._plurals.insert(0, trans.plural)

    def __getitem__(self, key):
        # First catalog wins for ALL lookups
        for cat in self._catalogs:
            try:
                return cat[key]
            except KeyError:
                pass
        raise KeyError(key)

The update() prepend is correct for plural lookups (each catalog needs its own plural function). But __getitem__ shouldn't give the prepended catalog priority for non-plural string lookups — these should follow the normal priority order (LOCALE_PATHS > INSTALLED_APPS > Django built-in).

Expected behavior

Non-plural gettext() lookups should respect the documented translation priority order regardless of plural form differences between catalogs.

Actual behavior

A third-party package with a cosmetically different Plural-Forms header silently overrides all matching non-plural translations from higher-priority sources.

Impact

In our project, this causes 28 translations to be silently wrong. The project has no way to fix this without either:

• Monkey-patching TranslationCatalog
• Patching the third-party package's .po files in the Docker build
• Adding pgettext context to every conflicting string

Environment

• Django 5.1 (also affects 5.0, likely all versions since #34221 was fixed)
• Python 3.12
• The issue was introduced/amplified by the fix for #34221 which changed update() to only merge with the top catalog