Blog Post
Claudius allensmith81 (wow, first time I've mentioned you both in over a decade - I miss working with you both on the daily!)
Localized Community is a feature that you enable. This creates an experience where you have language top-level categories and you can utilize a language picker to switch between languages.
A community with multiple languages may not want that type of experience, where they lock users into a top-level language category. This is where "Community language" setting comes into play.
With Localized Community enabled, we allow users to set their "Email language" that they prefer to receive notifications in. This setting is more for users to "override" if they like to browse in more than one language. Otherwise, this setting is going to adopt the language that you browse the community in by default.