The GNU convention for naming your locale(7) takes forms such as

ll
ll_cc
ll.enc
ll_cc.enc

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/.