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This property and the CultureInfo.GetCultureInfoByIetfLanguageTag(string) method are deprecated. Instead, you should use a the CultureInfo.Name property. IETF tags and names are identical.
The RFC 4646 standard that is maintained by the Internet Engineering Task Force (IETF) defines an IETF language tag, which provides a uniform means of identifying a language. The format of an IETF language tag is similar to the culture name returned by the CultureInfo.Name property, but does not identify a culture uniquely. That is, different cultures share the same IETF language tag if those cultures have identical linguistic characteristics. The linguistic characteristics of a culture are contained in the System.Globalization.TextInfo object associated with a System.Globalization.CultureInfo object.
An IETF language tag consists of a mandatory language identifier, an optional script identifier, and an optional region identifier.