public static Language fromLanguageTag(String languageTag) {
        if (languageTag == null) return UNKNOWN;
        return fromLocale(LocaleFactory.fromLanguageTag(languageTag));
    }
Convenience method for calling <code>fromLocale(LocaleFactory.fromLanguageTag(languageTag))</code>.
Returns UNKNOWN when passed null or an unknown language tag.

@param languageTag the language tag for which the <code>Language</code> to return
@return the corresponding <code>Language</code>, or {@link #UNKNOWN} if not known
linguistics/src/main/java/com/yahoo/language/Language.java