Get list ofTwoLetterISOLanguageName from list of EnglishName

Sounds like you just need to take the complete list of neutral (language-only) cultures, join that with your English names list, and then project that:

var languageCultures = CultureInfo.GetCultures(CultureTypes.NeutralCultures);
var twoLetterNames = from culture in languageCultures
                     join englishName in englishNames
                       on culture.EnglishName equals englishName
                     select culture.TwoLetterISOLanguageName;

If you do this regularly, you might want to build a Dictionary<string, string>:

var cultureByLanguage = languageCultures.ToDictionary(x => x.EnglishName);

or:

var twoLetterIsoByLanguage = languageCultures.ToDictionary
    (x => x.EnglishName,
     x => TwoLetterISOLanguageName);