ISO 639-1 is the first part of the ISO 639 international-standard language-code family. It consists of 136 two-letter codes used to identify the world's major languages. These codes are a useful international shorthand for indicating languages. For example:
The ISO 639-1 list became an official standard in 2002, but had existed in draft format for some years before. The last code added was ht, representing Haitian Creole on 2003-02-26. The use of the standard was encouraged by IETF language tags, introduced in RFC 1766 in March 1995, and continued by RFC 3066 from January 2001 and RFC 4646 from September 2006. Infoterm (International Information Center for Terminology) is the registration authority for ISO 639-1 codes.
New ISO 639-1 codes are not added if an ISO 639-2 code exists, so systems that use ISO 639-1 and 639-2 codes, with 639-1 codes preferred, do not have to change existing codes.
If an ISO 639-2 code that covers a group of languages is used, it may still be obsoleted for some data by a new ISO 639-1 code.
There is no specification on treatment of macrolanguages (see ISO 639-3).

English is represented by en
German is represented by de (from the endonym Deutsch)
Japanese is represented by ja (even though its endonym is Nihongo)