We could try to filter the `WPLANG` option:

    is_admin() && add_filter( 'option_WPLANG', function( $value )
    {
    	if( $lang = get_user_meta( get_current_user_id(), 'wpse_lang', true ) )
    		$value = $lang;
    	return $value;	
    } );	

where we check if the current user has the `wpse_lang` user meta key set, with values like `is_IS`, `da_DK`, ...

We might want to add restrictions so this will not affect the options settings page.

Then we could add the language selection for each user, on the user settings page. 

There we could use the `wp_dropdown_languages()` function, with the `get_available_languages()` function, to display the available language select-box.