BCP 47 is the IETF standard that defines language tags used in HTML lang attributes, hreflang links, and Content-Language headers. A tag is built from a primary language subtag plus optional script, region, variant, and extension subtags joined by hyphens.

What casing should a language tag use?

Casing is not significant for matching, but the conventional canonical form is lowercase language (en), Titlecase script (Hant), and uppercase region (HK). The tool outputs this canonical casing so your markup stays consistent.

When do I need a script subtag?

Add a script subtag when the language is written in more than one script and you need to distinguish them, such as zh-Hans for Simplified Chinese versus zh-Hant for Traditional. For most single-script languages the script is omitted.

Does a valid tag mean the subtags exist?

The validator checks structure — a 2 to 3 letter language, an optional 4-letter script, an optional 2-letter or 3-digit region. It also flags subtags that are not in its bundled registry subset, but the full IANA registry is the authority for whether a code is registered.

Is anything sent to a server?

No. The registry subset and the validation logic are bundled into the page and run in your browser, so lookups and validations are instant and private.

BCP 47 Language Tag Lookup & Validator

The BCP 47 Language Tag Lookup & Validator helps you choose and verify the value of an HTML lang attribute on multilingual pages. WCAG 3.1.1 (Language of Page) and 3.1.2 (Language of Parts) both depend on a correctly formed language tag, and a malformed tag silently breaks screen-reader pronunciation and hreflang targeting.

How it works

Parse into subtags. Your tag is split on hyphens. The first subtag must be a 2 to 3 letter primary language; an optional 4-letter script subtag and an optional 2-letter or 3-digit region subtag may follow, then any variant or extension subtags.
Validate each subtag. Each position is checked against the BCP 47 structure rules. Subtags that match the structure but are not in the bundled registry subset are flagged as warnings rather than errors, since the full IANA registry is larger.
Canonicalise. A well-formed tag is rebuilt in canonical casing — language lowercase, script Titlecase, region uppercase — for example zh-Hant-HK. The tool shows the exact lang="…" string to paste.
Search. A second box searches the bundled registry by subtag code or name so you can find, say, the code for “Georgian” or “Cyrillic”.

Tips and notes

Set lang on the root html element for the page default, and add lang on any element whose content is in a different language to satisfy 3.1.2.
Omit the region unless it is meaningful — en is usually better than guessing en-US when content is region-neutral.
Use the same canonical tags for your hreflang link annotations so search engines and assistive technology agree on the language.