WP Multilang

1. Set up your languages in WP Multilang → Languages.
2. Tick the same languages on the Languages tab and choose the same default language.
3. Add the switcher, and remove WP Multilang’s language switcher (widget, menu item or block) if you used one.

The plugin builds each link with WP Multilang’s own URL function, using the slug you gave each language there, and follows its setting for a folder on the default language. The current language comes from WP Multilang. WP Multilang prints its own hreflang links.