Rey is compatible with WPML , GTranslate and Polylang plugins, so you will first need to install either one of these too.
WPML is much more powerful, however Polylang is free. It’s your choice which one to use.
- Polylang tutorials: https://www.youtube.com/results?search_query=polylang
- WPML tutorials: https://www.youtube.com/results?search_query=wpml
*Rey is also compatible with QTranslateX but as far as i saw it’s discontinued.
Display the language switcher in the header
By default, the language switcher should be automatically shown in the header. But that only happens when you are using the Default Header in Customizer > Header > General > Layout option eg: https://d.pr/i/K6KqRl .
If the Layout is set on a Header – Global section, than you’re going to need to add the Header – Language Switcher element .
Quick video shown how to add the Language switcher element – https://d.pr/v/EYlJJo .