function getProviderIDsForLanguage(languageId) {
        if (!_providers[languageId]) {
            return [];
        }
        return _providers[languageId].map(function (provider) {
            return provider.name;
        });
    }
Returns an array of the IDs of providers registered for a specific language

@param {!string} languageId
@return {Array.<string>} Names of registered providers.