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ISO 3166-1(alpha-2)
A region designator is a code that represents a country (ISO 3166-1 standard, two-letter, capitalized code). To specify a specific dialect, use a hyphen to combine a language designator with a region designator. For example, to specify common English, use the en language designator as locale.
To specify the English language as it is used in the United Kingdom, use en-GB as the locale.