In ISO 639-1, the two-letter code for English is en.
ISO 639-1 is the language-identification part of the ISO 639 standards. It assigns two-letter codes to major languages, making them convenient for software, websites, libraries, and international data exchange. The code is written in lowercase, so English is represented as “en,” while Spanish is “es,” French is “fr,” and German is “de.”
The code should not be confused with English’s three-letter ISO 639-2 and ISO 639-3 code, eng. Nor is it a country code: the country-code top-level domain for England does not exist separately, and .uk identifies the United Kingdom. In web addresses and language tags, en may be extended with regional information, such as en-GB for British English or en-US for American English.