You can create fully functional multilingual site using one of the popular multilingual plugins supported by Rubenz theme. Those are WPML (powerful all-in-one paid plugin) and Polylang (lightweight alternative available for free).
When one of these plugins is setup and activated you will see a new widget area for the language switcher.
When WPML is activated the wizard will guide your on how to setup everything including the language switcher. In PolyLang you need to go to Appearance -> Widgets and drag & drop a language switcher to Language Switcher Area.
Language switcher is customizable. You can choose it to appear as dropdown or horizontal list. While the theme provides some styling for the switcher, all the customizations are available inside multi-lingual plugin settings.
Please note that AJAX transitions are disabled when you switch between the page languages. The reload is need for correct page display.