Hi There @petchgabriel
Thanks for writing in! X/Pro themes are compatible with WPML plugin (https://wpml.org/). It's a paid plugin and when you purchase their license, you can get their support as well when needed.
As for free plugins, qTranslate is one of the plugins that you can check (https://wordpress.org/plugins/qtranslate-x/). It is compatible with X/Pro, however you may experience issues according to your specific WordPress setup and so on. In such cases, you will need to escalate the issues through plugin developer.
Alternatively, you can also follow our translation guide here (https://theme.co/apex/forum/t/setup-translation/61).