The country code represents the region of the language as a 2 letter ISO code.
The IETF code represents the language as the two letter ISO code including the region (e.g. en-US).
ISO 2 Code
The ISO 2 letter code represents the language with two letters.
ISO 3 Code
The ISO 3 letter code represents the language with three letters.
The plural expression defines how to map a plural into the language index. This expression is used to determine the plural form for the translations.
Primary Of Group
The primary language of a group indicates whether a language is the primary language of a group of languages. The group is determine by the ISO 2 letter code.