Each language identifier is composed of a primary language identifier indicating the language and a sublanguage identifier indicating the country/region. The language identifier corresponds to a particular locale, for example, English (United States), represented as "en-US". The language identifier is used as part of the locale identifier.

This MSDN link includes a table of supported locales, what version of Windows each locale was introduced in, the hexadecimal locale identifier value.

Useful if you want to develop multi-lingual applications.