Hi Bruno,
The text you’re after lives in the content.php file. It’s on line 40 that looks like this:
<span class="comments-link"><?php comments_popup_link( __( 'Leave a comment', 'twentyfourteen' ), __( '1 Comment', 'twentyfourteen' ), __( '% Comments', 'twentyfourteen' ) ); ?></span>
Editing that line will change the text that’s included on posts. Do note that wordpress talks about comments elsewhere. If you want to change the word comment to something else throughout the site, that’s a far bigger job.
Hope this helps,
Simon