For your information, there is a small code missing inside "loop-portfolio-single.php".
I've translated all the theme in Italian, but the next and previous link in the single portfolio item was again in English.
So I've looked at the code and notice that the part 'avia_framework' was missing.
The correct code is:
<?php previous_post_link('<span class="previous_post_link">← %link </span><span class="post_link_text">'.__('(previous entry)', 'avia_framework'))."</span>"; ?>
<?php next_post_link('<span class="next_post_link"><span class="post_link_text">'.__('(next entry)', 'avia_framework').'</span> %link →</span>'); ?>
I hope this could help someone with this translation problem :-)