What is ISO 639-1?

ISO 639-1 is the standard that assigns two-letter codes to languages: en for English, es for Spanish, de for German, ja for Japanese. These codes are the most common way software identifies a language, and they form the primary subtag of BCP 47 language tags.

Coverage is deliberately limited: ISO 639-1 codes exist only for a few hundred major languages. The broader parts of the ISO 639 family use three-letter codes, ISO 639-2 and the comprehensive ISO 639-3, which covers thousands of living languages. The standard rule when building a tag is to use the shortest code available, so two-letter codes win whenever one exists.

A language code alone often underspecifies: pt could be European or Brazilian Portuguese, zh says nothing about Simplified versus Traditional script. That is why locale identifiers append region or script subtags. In Laravel, the directories under lang/ conventionally use ISO 639-1 codes, optionally extended with a region, like pt_BR.