https://learn.microsoft.com/ru-ru/dotnet/api/system.globalization.cultureinfo.twoletterisolanguagename?view=netframework-4.8.1
CultureInfo.TwoLetterISOLanguageName Property (System.Globalization) | Microsoft Learn
Gets the ISO 639-1 two-letter or ISO 639-3 three-letter code for the language of the current CultureInfo.
property systemglobalizationmicrosoftlearn