About IETF Language Tag Validator

Validate BCP 47 tag (zh-Hant-TW) — canonical form + regions. Part of the DevTools Surf developer suite. Browse more tools in the Text / String collection.

Use Cases

• i18n engineers validating language tags in translation config files
• Frontend developers setting correct lang attributes in HTML documents
• API developers validating Accept-Language headers from client requests
• CMS developers ensuring locale codes match BCP 47 standards

Tips

• Paste a BCP 47 tag like zh-Hant-TW to validate structure and region
• Check the canonical form output to normalize inconsistent tag casing
• Validate language tags before storing them in i18n config files

Fun Facts

• BCP 47 (Best Current Practice 47) combines RFC 5646 and RFC 4647 to define language tags. It supports over 8,000 languages from the ISO 639 registry.
• The subtag registry maintained by IANA contains over 9,000 entries including languages, scripts, regions, and variants — it is updated roughly every 2-4 weeks.
• The longest valid BCP 47 tag can theoretically exceed 35 characters, like 'sl-Latn-IT-nedis' (Slovenian, Latin script, Italy, Nadiza dialect).

FAQ

What is a BCP 47 language tag?

The IETF format for language identifiers: language-script-region-variant. Examples: `en`, `zh-Hant`, `zh-Hant-TW`, `de-CH-1901`. Used in HTML lang attributes and Accept-Language headers.

Does the validator suggest canonical forms?

Yes — it converts deprecated codes (iw → he, in → id) to modern ones and removes redundant script tags (zh-Hans-CN → zh-CN when Hans is default for CN).

Are region subtags always uppercase?

Yes per spec, but the validator accepts any case and normalizes on output. Language subtags lowercase, script title case, region uppercase.

What about private-use tags?

Supported — `x-` prefix marks private-use subtags. The validator accepts them but warns they're not interoperable outside your system.