When should I use 639-1 versus 639-2?

Use the two-letter 639-1 code (such as en) for HTML lang attributes, hreflang tags and most web APIs. Use the three-letter 639-2 code (eng) for libraries, bibliographic data and languages that have no two-letter code.

Which 639-2 variant is shown?

These are the 639-2/T (terminology) codes, which match the 639-3 codes most closely. A few languages also have a 639-2/B (bibliographic) variant, but 639-2/T is the form used in modern software.

Is anything sent to a server?

No. The language list ships with the page, so every search runs entirely in your browser.

ISO Language Codes Reference

Look up ISO 639-1 and 639-2 language codes and native names.

ISO 639 language codes

Look up any language by its ISO 639-1 two-letter code, ISO 639-2 three-letter code, English name or native name. Search any of those and the table filters instantly.

Use the two-letter code for lang attributes and web APIs, and the three-letter code where a longer code is required. All lookups run in your browser.