Choosing the backend language
You can choose the backend language (it’s independent of the default language) in the personal options sections of the Users → Your profile settings page. If you don’t choose one, the backend will be displayed in the WordPress default language.
Translating the user biographical info
You just have to fill the corresponding text fields in the user profile panel.
Filtering the content on backend side
Polylang adds a new menu in the admin bar which allows you to filter the content by language (posts, pages, media, categories, tags, comments, string translations). This choice is persistant (kept when you logout) and also impacts the default language when creating a new content. It has no influence on the language of the user admin interface, which can be chosen in the user profile.