What happen if you use the function __(
inside the function previous_post_link
?
<?php previous_post_link( '%link', __('<i class="icon-chevron-left"></i><span class="sub-title">Previous article</span><br />%title') ); ?>
After you need to rescan your theme in your wp-admin and then go in String Tranlation page to see if it's there.
Maybe it's not the solution but try it and tell me what it gives you.