Translation issue

When I use the following tag : [wpsc_create_ticket]
I have a new ticket form.

But a part of this form is not translatable.
Digging that issue I observed my translation is done in .po file but doesn’t work on the page.

For example the translation of “I’m not a robot” works
and the translation of “Short description of the ticket” doesn’t work

In fact I found “I’m not robot” in file:
includes/admin/tickets/create_ticket/load_create_ticket.php:75 in HTML/php code
<div style=”padding-top:3px;”><?php _e(“I’m not a robot”,’supportcandy’)?></div>

But “Short description” is in file :
“includes/class-wpsc-install.php:312” and it’s only in php
‘wpsc_tf_extra_info’, __(‘Short description of the ticket.’,’supportcandy’));

I use Loco Translate.
All texts not included in html code are not translatable.

Any idea ?