Frontend language per user

Thank you for interest to my theme. I have found solution of my task.

First of all the easiest way is to use a plugin, JSM-user-locale is good plugin for this task: wordpress.org/plugins/jsm-user-locale

Another way is to use standard WP functions.
You can get user locale with function

get_user_meta( $user_id, 'locale', $single = true );

Site localization you can check with

get_locale();

If you have to update user`s localization with function

update_user_meta( $user_id, 'locale', $user_locale );

And for switching localization you can use function:

switch_to_locale( $user_locale );

This function will switch localization for one session.

All above functions help updating user functions.

I hope this will be helpful for you.