flask-babel-locale-chain

Smart locale fallback chains for Flask-Babel -- because pt-BR users deserve pt-PT, not English.

The Problem

Flask-Babel's translation system falls back directly to the app's BABEL_DEFAULT_LOCALE when a regional locale variant is missing. There is no intermediate fallback.

Example: A user's browser sends Accept-Language: pt-BR. Your Flask app has pt-PT translations but no pt-BR locale. Flask-Babel skips pt-PT entirely and shows English (or whatever your BABEL_DEFAULT_LOCALE is).

The same thing happens with es-MX -> es, fr-CA -> fr, de-AT -> de, and every other regional variant.

Your users see English when a perfectly good translation exists in a sibling locale.

The Solution

One extension. Zero changes to your existing translation code.

flask-babel-locale-chain wraps Flask-Babel with fallback chain support using Python's native gettext.GNUTranslations.add_fallback mechanism. Missing keys in the primary locale catalogue are resolved from fallback locales before reaching the app's default language. Your existing translation calls just work:

• _("key") and gettext("key") in Python code
• {{ _("key") }} in Jinja2 templates
• ngettext() pluralization
• All Flask-Babel translation functions

Installation

pip install flask-babel-locale-chain

Quick Start

1. Install the package

pip install flask-babel-locale-chain

2. Initialize the extension

from flask import Flask
from flask_locale_chain import LocaleChainBabel

app = Flask(__name__)
lcb = LocaleChainBabel(app)

That's it. All 75 default fallback chains are active. A pt-BR user will now see pt-PT translations when pt-BR is not available.

3. (Optional) Add custom chains via Flask config

# config.py or in your app factory

app.config["LOCALE_FALLBACK_CHAINS"] = {
    "pt-BR": ["pt-PT", "pt"],
    "es-MX": ["es-419", "es"],
    "fr-CA": ["fr"],
}

Your custom chains are merged with the defaults. Keys you specify replace the corresponding default chain.

4. Use _() / gettext() as normal

from flask_babel import gettext as _

@app.route("/")
def index():
    return _("greeting")  # Falls back through the chain automatically

App Factory Pattern

flask-babel-locale-chain supports Flask's application factory pattern with init_app():

from flask import Flask
from flask_locale_chain import LocaleChainBabel

lcb = LocaleChainBabel()

def create_app():
    app = Flask(__name__)
    app.config["BABEL_DEFAULT_LOCALE"] = "en"
    app.config["LOCALE_FALLBACK_CHAINS"] = {
        "pt-BR": ["pt-PT", "pt"],
    }
    lcb.init_app(app)
    return app

You can also pass an existing Babel instance if you need to configure Babel separately:

from flask_babel import Babel
from flask_locale_chain import LocaleChainBabel

babel = Babel()
lcb = LocaleChainBabel()

def create_app():
    app = Flask(__name__)
    babel.init_app(app)
    lcb.init_app(app, babel=babel)
    return app

Configuration Modes

Default (zero config)

Just create the extension. Uses all 75 built-in fallback chains covering Chinese, Portuguese, Spanish, French, German, Italian, Dutch, English, Arabic, Norwegian, and Malay regional variants.

lcb = LocaleChainBabel(app)

Flask config

# Custom chains merged with defaults
app.config["LOCALE_FALLBACK_CHAINS"] = {
    "pt-BR": ["pt-PT", "pt"],
    "ja-JP": ["ja"],
}

Programmatic API

from flask_locale_chain import configure, reset

# Zero-config -- all 75 default chains
configure()

# Override specific chains while keeping all defaults
configure(overrides={"pt-BR": ["pt"]})

# Full custom map, merged with defaults
configure(fallbacks={"ja-JP": ["ja"]})

# Full custom map, no defaults
configure(fallbacks={"pt-BR": ["pt-PT"]}, merge_defaults=False)

# Restore default behaviour
reset()

API Reference

LocaleChainBabel(app=None, babel=None)

Flask extension class.

Parameter	Type	Default	Description
app	Flask | None	None	Flask application. If provided, init_app() is called immediately.
babel	Babel | None	None	Existing Babel instance. A new one is created if not provided.

init_app(app, babel=None)

Initialize the extension with a Flask application. Reads LOCALE_FALLBACK_CHAINS from app.config and merges with built-in defaults.

get_chain(locale) -> list[str]

Return the fallback chain for a given locale code. Returns an empty list if no chain is configured.

configure(overrides=None, fallbacks=None, merge_defaults=True, default_locale="en")

Activate chain-aware translation lookup (programmatic API).

Parameter	Type	Default	Description
overrides	dict | None	None	Additional or replacement chains merged on top of defaults
fallbacks	dict | None	None	A complete fallback map
merge_defaults	bool	True	Whether to include defaults when fallbacks is supplied
default_locale	str	"en"	The project's default locale

reset()

Remove all custom configuration and restore default behaviour. Safe to call multiple times.

DEFAULT_FALLBACKS

A dict[str, list[str]] containing all 75 built-in fallback chains. Importable for inspection or as a base for custom maps.

merge_fallbacks(overrides=None, base=None, merge_defaults=True)

Merge two fallback maps, returning a new dict. Entries in overrides replace same-key entries in base. Neither input is mutated.

Flask Config Reference

Setting	Type	Description
LOCALE_FALLBACK_CHAINS	dict[str, list[str]]	Custom fallback chains, merged with built-in defaults

Priority order (highest to lowest):

1. configure() call (programmatic API)
2. LOCALE_FALLBACK_CHAINS Flask config setting
3. Built-in defaults (zero-config)

Default Fallback Map

Chinese (Traditional)

Locale	Fallback Chain
zh-Hant-HK	zh-Hant-TW -> zh-Hant -> (default)
zh-Hant-MO	zh-Hant-HK -> zh-Hant-TW -> zh-Hant -> (default)
zh-Hant-TW	zh-Hant -> (default)

Chinese (Simplified)

Locale	Fallback Chain
zh-Hans-SG	zh-Hans -> (default)
zh-Hans-MY	zh-Hans -> (default)

Portuguese

Locale	Fallback Chain
pt-BR	pt-PT -> pt -> (default)
pt-PT	pt -> (default)
pt-AO	pt-PT -> pt -> (default)
pt-MZ	pt-PT -> pt -> (default)

Spanish

Locale	Fallback Chain
es-419	es -> (default)
es-MX	es-419 -> es -> (default)
es-AR	es-419 -> es -> (default)
es-CO	es-419 -> es -> (default)
es-CL	es-419 -> es -> (default)
es-PE	es-419 -> es -> (default)
es-VE	es-419 -> es -> (default)
es-EC	es-419 -> es -> (default)
es-GT	es-419 -> es -> (default)
es-CU	es-419 -> es -> (default)
es-BO	es-419 -> es -> (default)
es-DO	es-419 -> es -> (default)
es-HN	es-419 -> es -> (default)
es-PY	es-419 -> es -> (default)
es-SV	es-419 -> es -> (default)
es-NI	es-419 -> es -> (default)
es-CR	es-419 -> es -> (default)
es-PA	es-419 -> es -> (default)
es-UY	es-419 -> es -> (default)
es-PR	es-419 -> es -> (default)

French

Locale	Fallback Chain
fr-CA	fr -> (default)
fr-BE	fr -> (default)
fr-CH	fr -> (default)
fr-LU	fr -> (default)
fr-MC	fr -> (default)
fr-SN	fr -> (default)
fr-CI	fr -> (default)
fr-ML	fr -> (default)
fr-CM	fr -> (default)
fr-MG	fr -> (default)
fr-CD	fr -> (default)

German

Locale	Fallback Chain
de-AT	de -> (default)
de-CH	de -> (default)
de-LU	de -> (default)
de-LI	de -> (default)

Italian

Locale	Fallback Chain
it-CH	it -> (default)

Dutch

Locale	Fallback Chain
nl-BE	nl -> (default)

English

Locale	Fallback Chain
en-GB	en -> (default)
en-AU	en-GB -> en -> (default)
en-NZ	en-AU -> en-GB -> en -> (default)
en-IN	en-GB -> en -> (default)
en-CA	en -> (default)
en-ZA	en-GB -> en -> (default)
en-IE	en-GB -> en -> (default)
en-SG	en-GB -> en -> (default)

Arabic

Locale	Fallback Chain
ar-SA	ar -> (default)
ar-EG	ar -> (default)
ar-AE	ar -> (default)
ar-MA	ar -> (default)
ar-DZ	ar -> (default)
ar-IQ	ar -> (default)
ar-KW	ar -> (default)
ar-QA	ar -> (default)
ar-BH	ar -> (default)
ar-OM	ar -> (default)
ar-JO	ar -> (default)
ar-LB	ar -> (default)
ar-TN	ar -> (default)
ar-LY	ar -> (default)
ar-SD	ar -> (default)
ar-YE	ar -> (default)

Norwegian

Locale	Fallback Chain
nb	no -> (default)
nn	nb -> no -> (default)

Malay

Locale	Fallback Chain
ms-MY	ms -> (default)
ms-SG	ms -> (default)
ms-BN	ms -> (default)

Example

A complete working example is included in the example/ directory. It demonstrates locale fallback chains with Portuguese (pt-BR -> pt-PT -> pt) and Chinese (zh-Hant-HK -> zh-Hant-TW -> zh-Hant) variants.

cd example && pip install -r requirements.txt && python app.py

Then visit http://localhost:5000/?lang=pt-BR to see the fallback in action.

How It Works

1. LocaleChainBabel wraps Flask-Babel's Babel extension and reads LOCALE_FALLBACK_CHAINS from your Flask config.
2. The resolved fallback chains are merged with the 75 built-in defaults.
3. When a translation key is missing in the active locale, the chain is walked in order -- each fallback locale's catalogue is checked before reaching the app's default locale.
4. The fallback resolution uses Python's native gettext.GNUTranslations.add_fallback mechanism -- no monkey-patching.
5. Your existing _() / gettext() / ngettext() calls work unchanged.

FAQ

Is this production-ready? Yes. The library uses Python's native gettext.GNUTranslations.add_fallback mechanism and Flask-Babel's public API. No monkey-patching, no private API access.

Performance impact? Negligible. Fallback catalogues are loaded once per locale. After the initial wiring, gettext() calls walk the fallback chain with zero additional overhead from this library -- it is Python's built-in gettext resolution.

Does it work with .po and .mo files? Yes. This library operates on Flask-Babel's translation catalogues, which are compiled from .po files into .mo files. Any translation format that Flask-Babel supports will work.

Can I use a non-English default locale? Yes. The fallback chains are independent of your app's BABEL_DEFAULT_LOCALE. They only control which sibling locales are checked before the default language.

Can I deactivate it? Yes. Call reset() to remove all custom configuration, or simply remove the LocaleChainBabel extension from your app.

Does it work with Flask blueprints? Yes. Flask-Babel's translation system is app-wide, so fallback chains apply to all blueprints registered on the app.

Does it work with the app factory pattern? Yes. Use lcb = LocaleChainBabel() and then lcb.init_app(app) in your factory function.

Minimum Python version? Python 3.8. Works with Flask 2.0+ and Flask-Babel 3.0+.

Contributing

• Open issues for bugs or feature requests.
• PRs welcome, especially for adding new locale fallback chains.
• Run tests with: pytest

License

MIT License - see LICENSE file.

Built by i18nagent.ai