id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36980	TranslationCatalog.__getitem__ gives wrong priority to non-plural translations when plural forms mismatch	UHHHHHHHHHHHHHH		"= 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`:

{{{#!python
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
"	Bug	closed	Internationalization	5.1	Normal	worksforme	i18n translation plural TranslationCatalog		Unreviewed	0	0	0	0	0	0