I want my wordpress to have multiple UI languages while keeping my content, ie. posts, unchanged.

If a user, who may not be an admin, whose Accept-Language HTTP header prefers German, my site returns German interface while my posts are still in English. Or, there can be a Choose Language dropdown, which only changes the user interface.

I believe it's completely feasible as the admin panel already has Site Language option, but it just doesn't change according to user preference.

I searched multiple sites with keywords like "wordpress site language bilingual", "wordpress UI bilingual". They give me results like Multisite Language Switcher, Polylang. These plugins seem to manage/translate posts.

What's the correct way to provide bilingual for interface for visitors?