diff --git a/.sampo/changesets/centralize-default-locale.md b/.sampo/changesets/centralize-default-locale.md
new file mode 100644
index 0000000..1718cf5
--- /dev/null
+++ b/.sampo/changesets/centralize-default-locale.md
@@ -0,0 +1,8 @@
+---
+pypi/generaltranslation: patch
+pypi/gt-fastapi: patch
+pypi/gt-flask: patch
+pypi/generaltranslation-intl-messageformat: patch
+---
+
+Centralize hardcoded `"en"` locale defaults into a `LIBRARY_DEFAULT_LOCALE` constant. No behavioral change.
diff --git a/packages/generaltranslation-intl-messageformat/src/generaltranslation_intl_messageformat/_formatter.py b/packages/generaltranslation-intl-messageformat/src/generaltranslation_intl_messageformat/_formatter.py
index a81890f..d787137 100644
--- a/packages/generaltranslation-intl-messageformat/src/generaltranslation_intl_messageformat/_formatter.py
+++ b/packages/generaltranslation-intl-messageformat/src/generaltranslation_intl_messageformat/_formatter.py
@@ -11,6 +11,8 @@
 from babel.numbers import format_decimal
 from generaltranslation_icu_messageformat_parser import Parser
 
+LIBRARY_DEFAULT_LOCALE = "en"
+
 _parser = Parser()
 
 
@@ -27,13 +29,13 @@ class IntlMessageFormat:
         mf.format({"count": 5})  # "5 items"
     """
 
-    def __init__(self, pattern: str, locale: str = "en") -> None:
+    def __init__(self, pattern: str, locale: str = LIBRARY_DEFAULT_LOCALE) -> None:
         self._pattern = pattern
         self._ast = _parser.parse(pattern)
         try:
             self._locale = Locale.parse(locale, sep="-")
         except (UnknownLocaleError, ValueError):
-            self._locale = Locale("en")
+            self._locale = Locale(LIBRARY_DEFAULT_LOCALE)
 
     @property
     def pattern(self) -> str:
diff --git a/packages/generaltranslation/src/generaltranslation/formatting/_helpers.py b/packages/generaltranslation/src/generaltranslation/formatting/_helpers.py
index a88cb4c..bdad80a 100644
--- a/packages/generaltranslation/src/generaltranslation/formatting/_helpers.py
+++ b/packages/generaltranslation/src/generaltranslation/formatting/_helpers.py
@@ -9,6 +9,8 @@
 from babel import Locale
 from babel.core import UnknownLocaleError
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
+
 
 def _resolve_babel_locale(locales: str | list[str] | None = None) -> Locale:
     """Convert BCP 47 locale(s) to a Babel :class:`Locale` instance.
@@ -22,7 +24,7 @@ def _resolve_babel_locale(locales: str | list[str] | None = None) -> Locale:
         A :class:`babel.Locale` instance.
     """
     if locales is None:
-        return Locale("en")
+        return Locale(LIBRARY_DEFAULT_LOCALE)
     if isinstance(locales, str):
         locales = [locales]
     for tag in locales:
@@ -30,7 +32,7 @@ def _resolve_babel_locale(locales: str | list[str] | None = None) -> Locale:
             return Locale.parse(tag, sep="-")
         except (UnknownLocaleError, ValueError):
             continue
-    return Locale("en")
+    return Locale(LIBRARY_DEFAULT_LOCALE)
 
 
 def _get_language_code(locales: str | list[str] | None = None) -> str:
diff --git a/packages/generaltranslation/src/generaltranslation/locales/_get_locale_name.py b/packages/generaltranslation/src/generaltranslation/locales/_get_locale_name.py
index 91402eb..ee4a08b 100644
--- a/packages/generaltranslation/src/generaltranslation/locales/_get_locale_name.py
+++ b/packages/generaltranslation/src/generaltranslation/locales/_get_locale_name.py
@@ -8,12 +8,13 @@
 
 from babel import Locale
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales._types import CustomMapping
 
 
 def get_locale_name(
     locale: str,
-    default_locale: str | None = "en",
+    default_locale: str | None = LIBRARY_DEFAULT_LOCALE,
     custom_mapping: CustomMapping | None = None,
 ) -> str:
     """Return the display name of *locale* rendered in *default_locale*.
diff --git a/packages/generaltranslation/src/generaltranslation/locales/_get_locale_properties.py b/packages/generaltranslation/src/generaltranslation/locales/_get_locale_properties.py
index ac995dd..5546bd4 100644
--- a/packages/generaltranslation/src/generaltranslation/locales/_get_locale_properties.py
+++ b/packages/generaltranslation/src/generaltranslation/locales/_get_locale_properties.py
@@ -10,6 +10,7 @@
 from babel import Locale
 from babel.core import get_global
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales._types import CustomMapping, LocaleProperties
 
 _likely_subtags: dict[str, str] = dict(get_global("likely_subtags"))
@@ -105,7 +106,7 @@ def _build_component_name(
 
 def get_locale_properties(
     locale: str,
-    default_locale: str | None = "en",
+    default_locale: str | None = LIBRARY_DEFAULT_LOCALE,
     custom_mapping: CustomMapping | None = None,
 ) -> LocaleProperties:
     """Return a :class:`LocaleProperties` for *locale*."""
diff --git a/packages/generaltranslation/src/generaltranslation/locales/_get_plural_form.py b/packages/generaltranslation/src/generaltranslation/locales/_get_plural_form.py
index c28cc3c..1d84912 100644
--- a/packages/generaltranslation/src/generaltranslation/locales/_get_plural_form.py
+++ b/packages/generaltranslation/src/generaltranslation/locales/_get_plural_form.py
@@ -11,6 +11,7 @@
 
 from babel import Locale
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales._types import PLURAL_FORMS, PluralType
 
 # Aliases between our custom form names and CLDR form names
@@ -54,7 +55,7 @@ def get_plural_form(
     if forms is None:
         forms = list(PLURAL_FORMS)
     if locales is None:
-        locales = ["en"]
+        locales = [LIBRARY_DEFAULT_LOCALE]
 
     forms_set = set(forms)
 
@@ -95,7 +96,7 @@ def _get_cldr_category(n: int | float, locales: list[str]) -> str:
             continue
     # Default to English rules
     try:
-        rule = Locale("en").plural_form
+        rule = Locale(LIBRARY_DEFAULT_LOCALE).plural_form
         return rule(abs(n))
     except Exception:
         return "other"
diff --git a/packages/generaltranslation/src/generaltranslation/locales/_get_region_properties.py b/packages/generaltranslation/src/generaltranslation/locales/_get_region_properties.py
index a8cca79..2c0cc4f 100644
--- a/packages/generaltranslation/src/generaltranslation/locales/_get_region_properties.py
+++ b/packages/generaltranslation/src/generaltranslation/locales/_get_region_properties.py
@@ -9,13 +9,14 @@
 
 from babel import Locale
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales._get_locale_emoji import DEFAULT_EMOJI, EMOJIS
 from generaltranslation.locales._types import CustomRegionMapping
 
 
 def get_region_properties(
     region: str,
-    default_locale: str | None = "en",
+    default_locale: str | None = LIBRARY_DEFAULT_LOCALE,
     custom_mapping: CustomRegionMapping | None = None,
 ) -> dict[str, str]:
     """Return metadata for a region code.
diff --git a/packages/generaltranslation/src/generaltranslation/locales/_is_valid_locale.py b/packages/generaltranslation/src/generaltranslation/locales/_is_valid_locale.py
index 3538076..d6c6775 100644
--- a/packages/generaltranslation/src/generaltranslation/locales/_is_valid_locale.py
+++ b/packages/generaltranslation/src/generaltranslation/locales/_is_valid_locale.py
@@ -9,6 +9,7 @@
 from babel import Locale
 from babel.core import UnknownLocaleError
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales._types import CustomMapping
 
 # Scripts that are valid but may not be recognised by all display-name APIs.
@@ -109,7 +110,7 @@ def is_valid_locale(
     # For region codes, verify they're known
     if parsed.territory:
         try:
-            display_locale = Locale("en")
+            display_locale = Locale(LIBRARY_DEFAULT_LOCALE)
             if parsed.territory not in display_locale.territories:
                 return False
         except Exception:
@@ -119,7 +120,7 @@ def is_valid_locale(
     if parsed.script:
         if parsed.script not in SCRIPT_EXCEPTIONS:
             try:
-                display_locale = Locale("en")
+                display_locale = Locale(LIBRARY_DEFAULT_LOCALE)
                 if parsed.script not in display_locale.scripts:
                     return False
             except Exception:
diff --git a/packages/gt-fastapi/src/gt_fastapi/_setup.py b/packages/gt-fastapi/src/gt_fastapi/_setup.py
index 3f0fcf6..4717ea6 100644
--- a/packages/gt-fastapi/src/gt_fastapi/_setup.py
+++ b/packages/gt-fastapi/src/gt_fastapi/_setup.py
@@ -6,6 +6,7 @@
 from contextlib import asynccontextmanager
 from typing import Any
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales import determine_locale
 from gt_i18n import I18nManager, set_i18n_manager, t  # noqa: F401
 
@@ -45,7 +46,7 @@ def _detect_from_accept_language(request: Any, manager: I18nManager) -> str:
 def initialize_gt(
     app: Any,
     *,
-    default_locale: str = "en",
+    default_locale: str = LIBRARY_DEFAULT_LOCALE,
     locales: list[str] | None = None,
     project_id: str | None = None,
     cache_url: str | None = None,
diff --git a/packages/gt-flask/src/gt_flask/_setup.py b/packages/gt-flask/src/gt_flask/_setup.py
index 5268f58..0cf17ed 100644
--- a/packages/gt-flask/src/gt_flask/_setup.py
+++ b/packages/gt-flask/src/gt_flask/_setup.py
@@ -6,6 +6,7 @@
 from collections.abc import Callable
 from typing import Any
 
+from generaltranslation._settings import LIBRARY_DEFAULT_LOCALE
 from generaltranslation.locales import determine_locale
 from gt_i18n import I18nManager, set_i18n_manager, t  # noqa: F401
 
@@ -47,7 +48,7 @@ def _detect_from_accept_language(request: Any, manager: I18nManager) -> str:
 def initialize_gt(
     app: Any,
     *,
-    default_locale: str = "en",
+    default_locale: str = LIBRARY_DEFAULT_LOCALE,
     locales: list[str] | None = None,
     project_id: str | None = None,
     cache_url: str | None = None,