Fix standalone month alias cache leakage

Author: Funan Zhou

babel/localedata.py

@@ -269,7 +269,7 @@ def __getitem__(self, key: str | int | None) -> Any:
             val = alias.resolve(self.base).copy()
             merge(val, others)
         if isinstance(val, dict):  # Return a nested alias-resolving dict
-            val = LocaleDataDict(val, base=self.base)
+            return LocaleDataDict(val, base=self.base)
         if val is not orig:
             self._data[key] = val
         return val

tests/test_dates.py

@@ -16,7 +16,7 @@
 import freezegun
 import pytest
 
-from babel import Locale, dates
+from babel import Locale, dates, localedata
 from babel.dates import NO_INHERITANCE_MARKER, UTC, _localize, parse_pattern
 from babel.util import FixedOffsetTimezone
 
@@ -561,6 +561,23 @@ def test_format_date():
             "Sun, Apr 1, '07")
 
 
+@pytest.mark.parametrize(
+    ('locale', 'expected'),
+    [
+        ('de', 'Oktober'),
+        ('he', 'אוקטובר'),
+        ('no', 'oktober'),
+        ('fr', 'octobre'),
+    ],
+)
+def test_format_date_standalone_month_name_isolation(locale, expected):
+    localedata._cache.clear()
+    d = date(2025, 10, 1)
+
+    assert dates.format_date(d, 'LLLL', locale='he') == 'אוקטובר'
+    assert dates.format_date(d, 'LLLL', locale=locale) == expected
+
+
 def test_format_datetime(timezone_getter):
     dt = datetime(2007, 4, 1, 15, 30)
     assert (dates.format_datetime(dt, locale='en_US') ==