---
id: compare-locales
version: "9.0.5"
license: MPL-2.0
license_treatment: copyleft
maintenance: active
---

# compare-locales — Lint Mozilla localizations

License: copyleft · Maintenance: active · Downloads: 81.7K/mo

## What it is and what it does

compare-locales is a command-line linter and merge tool for Mozilla localization workflows. It validates localization files against a reference (typically English) to detect missing strings, obsolete entries, and runtime errors without false positives, while also warning on potential issues. The package includes two main tools: compare-locales for validating locales across a project, and moz-l10n-lint for checking local work and detecting string conflicts.

The tool is configured via an l10n.toml project file and works with Mozilla's localization structure. It can merge localizations by padding them with missing English strings and replacing error-prone entities with English fallbacks, making it essential for maintaining consistency across multiple language variants in large projects.

Use it for:

- Validate all locales in a Mozilla project against a reference configuration to catch missing or obsolete strings before release.
- Lint local localization work before committing to detect errors and conflicts in string definitions.
- Merge localizations with English fallbacks to ensure no untranslated strings break the application at runtime.
- Check for conflicts between locally-defined strings and those already exposed to the l10n community.
- Integrate into CI/CD pipelines to enforce localization quality standards across monolithic and gecko-based projects.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Lints Mozilla localization files to find missing, obsolete, and erroneous strings; also merges localizations with English fallbacks and checks for conflicts in string definitions.

Yes, if you maintain Mozilla localizations or similar multi-language projects. The tool is actively maintained, has no known vulnerabilities, and low install friction. The copyleft license is standard for Mozilla tooling; verify it aligns with your project's licensing model before use.

## Install

pip install compare-locales
uv add compare-locales
poetry add compare-locales

## Installing compare-locales

Before you install: Low install friction with a pure-Python wheel distribution. Actively maintained as of May 2026, with recent commits and production-stable status.

License in practice: Mozilla Public License 2.0 (copyleft) requires that modifications and derivative works remain under MPL-2.0; suitable for internal tooling and open-source projects but check your licensing requirements if distributing modified versions.

Quickstart:

pip install compare-locales

compare-locales l10n.toml .
moz-l10n-lint l10n.toml

Requires an l10n.toml configuration file in your project; see moz-l10n-config documentation for format details.

Verify before relying:

- Whether fluent.syntax, six, and toml are bundled or require separate installation in your environment.
- Performance characteristics when linting large localization projects with many locales.

## Package facts

- License: MPL-2.0 (copyleft)
- Python support: supports_current
- Install friction: low
- Maintenance: active
- Downloads: 81.7K/month (top 15,000 on PyPI)
- Known vulnerabilities: none known

## Tags

localization linting, l10n validation, mozilla locales checker, translation string errors, l10n merge tool, localization quality assurance, l10n-tooling, mozilla-ecosystem, ci-cd-ready

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