Locale is the End-User
's Locale, represented as a BCP 47 RFC 5646
This is typically an ISO 639-1 Alpha-2 ISO 639‑1 Language Code in lowercase and an ISO 3166-1 alpha-2 Country Code in uppercase, separated by a dash.
As a compatibility note, some implementations have used an underscore as the separator rather than a dash, for example, en_US; Relying Parties MAY
choose to accept this locale syntax as well.
There might be more information for this subject on one of the following: