The GNU convention for naming your locale(7) takes forms such as
where ll is an ISO 639 language code (lowercase), cc is an ISO 3166 country code (uppercase), and enc is an encoding (uppercase). For example
en_GB -- British English
en_NZ.UTF-8 -- New Zealand English, UTF-8 encoding
ja -- Japanese language
ja.SJIS -- Japanese language, Shift-JIS encoding
ja_JP.UTF-8 -- Japanese language, UTF-8 encoding, as used in Japan
There is also a special locale, just named “C”, which just means “no localization”.
These names are used as directory names for holding the corresponding localization information in /usr/share/X11/locale/localename/.