localediff

Find drift between your i18n locale files — before your users do. You add a string to en.json, ship it, and three weeks later notice fr.json and zh.json were never updated. localediff catches that in CI: missing keys, plural forms a language actually needs, and keys that exist but were left blank. Framework-agnostic, zero dependencies.

pip install localediff
localediff ./locales

✗ fr.json
  missing (1): auth.errors.locked
  plural  cart.items — has {other}, missing {one}
  empty   (1): footer.copyright
  extra   (1): legacy.banner

✓ zh.json — in sync

✗ 1 of 2 file(s) drifted — 1 missing, 1 plural gap(s), 1 empty, 1 extra

Why another i18n tool

Most existing tools are tied to one framework, need an account, or are heavy AST-based linters. localediff just reads JSON. It compares structure, so it works for next-intl, react-intl, i18next, vue-i18n, Django/Flask JSON catalogs, or any plain message file — and it ships for both PyPI and npm with identical behavior.

It understands plurals per language

This is the part naive "diff two JSON files" scripts get wrong. English has two plural forms (one, other); Chinese has one (other); Russian has four (one, few, many, other). A file with only items_other is correct for Chinese but broken for French. localediff resolves the required CLDR categories from each target's language, so it flags the real bug without crying wolf on zh.json.

# en base: items_one + items_other
zh.json  →  items_other only   →  ✓ in sync   (Chinese needs only `other`)
fr.json  →  items_other only   →  ✗ missing {one}
ru.json  →  items_one + _other →  ✗ missing {few, many}

Usage

# Scan a folder. The base defaults to en.json; everything else is checked.
localediff ./locales

# Pick a different base language in the folder.
localediff ./locales --base de

# Compare specific files explicitly.
localediff --base en.json --check fr.json zh.json

# Shorthand: first file is the base.
localediff en.json fr.json zh.json

# Machine-readable output for CI gates.
localediff ./locales --format json

You can also run it as a module: python -m localediff ./locales.

What it checks

Nested objects are flattened to dot-paths (auth.errors.locked); arrays are indexed (steps.0). Plural keys use the i18next suffix convention (_zero, _one, _two, _few, _many, _other). Unknown languages fall back to parity with the base, so you never get a confidently-wrong result.

Options

--base <file|lang>    base/source locale (a file, or a lang stem in dir mode)
--check <files...>    one or more target locales to compare against the base
--dir <dir>           scan a directory of *.json locales
--lang <code>         force the target language for plural rules
--format text|json    output format (default: text)
--ignore-missing      don't report missing keys
--ignore-extra        don't report extra keys
--ignore-plural       don't report plural gaps
--ignore-empty        don't report empty values
-v, --version
-h, --help

In CI

localediff exits non-zero when anything has drifted:

Also available for Node

npx localediff ./locales

Same checks, same flags, same exit codes — localediff on npm.

Scope

JSON locale files only (the common case). YAML/.properties/gettext are not supported — parsing them would mean pulling in a dependency, and zero-dep is the point. Convert to JSON, or open an issue to discuss.

License

MIT