How to add a translatable string to post date

You should use placeholders:

$time = sprintf( 
    '<i class="fa fa-clock-o" aria-hidden="true"></i> %s: 
     <time datetime="%s">%s</time>', 
     esc_html__('Posted on', 'textdomain'), 
     get_the_time('Y-m-d'), 
     get_the_time('j F Y') 
);