set language(lang) {
        if (!this.i18next) {
            return;
        }
        this.options.language = lang;
        if (this.i18next.language === lang) {
            return;
        }
        this.i18next.changeLanguage(lang, (err) => {
            if (err) {
                return;
            }
            this.rebuild();
            this.emit('languageChanged');
        });
    }
Sets the language for this form.
@param {string} lang - The language to use (e.g. 'en', 'sp', etc.)