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.
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 .