This will give you all possible languages (I get 354 of them)
var regionalLanguages = CultureInfo.GetCultures(CultureTypes.AllCultures).Select(c => c.DisplayName);
This will give you all possible languages with specific regions (I get 210 of them)
var regionalLanguages = CultureInfo.GetCultures(CultureTypes.SpecificCultures).Select(c => c.DisplayName);