The ISO 639-1 two-letter language code. It can be optionally qualified by an ISO 3166-1 two-letter country code. The language code or qualified language-country code must be a valid registered code, for example: en (English), en-US (English-United States) and en-GB (English-United Kingdom). This field is case sensitive. Values must be entered in the correct case (for example: en-US).
The ISO language code is commonly used in Web applications and when interfacing with external libraries that support localization.