i18n Translation File Format Comparison:
JSON vs YAML vs TOML

When building a multilingual app, choosing the right translation file format is a more important decision than it may seem. This guide compares JSON, YAML, and TOML in the context of i18n development.

Summary Comparison Table

Criteria JSON YAML TOML
Readability △ Medium ◎ High ○ High
Comments ✗ Not supported ✓ Supported ✓ Supported
Nesting ○ Free-form ◎ Free-form, concise △ Section-based
Tooling support ◎ Best-in-class ○ Extensive △ Limited
React i18next ◎ Standard ○ Supported ✗ Non-standard
Vue i18n ◎ Standard ○ Supported △ Plugin required
Flutter ◎ ARB format (JSON extension) ✗ Non-standard ✗ Non-standard
null values ✓ Supported ✓ Supported ✗ Not supported
Multi-line strings △ \n escape only ◎ Block style ○ Triple quotes
Crowd translation SaaS ◎ Universal support ○ Widely supported △ Limited

Format Details

JSON

JSON

✓ Best tooling support of any format

✓ Standard for all i18n libraries

✓ Fast and stable parsers

✗ No comments

✗ No trailing commas allowed

✗ Verbose multi-line strings

YAML

YAML

✓ Comments for context annotations

✓ Visually readable indentation

✓ Easy multi-line strings

✗ Indentation errors can corrupt files

✗ Implicit type coercion (yes → true, etc.)

TOML

TOML

✓ Explicit type definitions

✓ Excellent for config files

✗ Rarely used as an i18n standard

✗ No null support

✗ Deep nesting becomes complex

The Same Translation Data in All Three Formats

JSON

{
  "greeting": "Hello, {{name}}!",
  "nav": {
    "home": "Home",
    "about": "About Us"
  },
  "errors": {
    "required": "This field is required.",
    "email": "Please enter a valid email."
  }
}

YAML

# Common translation file - English
greeting: "Hello, {{name}}!"

nav:
  home: Home
  about: About Us  # Header navigation

errors:
  required: This field is required.
  email: Please enter a valid email.

TOML

greeting = "Hello, {{name}}!"

[nav]
home = "Home"
about = "About Us"

[errors]
required = "This field is required."
email = "Please enter a valid email."

Which Format Should You Choose?

When in doubt, choose JSON

All major frameworks — i18next, Vue i18n, Angular i18n, Flutter ARB — support JSON as the standard. Cloud translation SaaS platforms like Phrase, Crowdin, and Lokalise also offer the best JSON support. If you're not sure which to pick, JSON is the safe default.

Choose YAML if you need comments

If you want to annotate context for translators (non-engineers), YAML is a strong option thanks to its # comment support. i18next supports YAML directly via i18next-parser.

Avoid TOML for i18n

TOML is excellent for Rust/Go config files, but deep nesting becomes unreadable in translation files, and its lack of null support can be a problem in i18n workflows. Stick to JSON or YAML for localization.

Need to Convert Between JSON and YAML?

If you need to migrate your existing project to a different format, try JSON Converter Tools for browser-based JSON↔YAML conversion (also server-free).

Check your translation file key mismatches now

Use the Diff Checker → JSON Converter Tool