Hi Grégoire,
This is actually a piece of text generated by WooCommerce itself rather than by WooCommerce Tickets so you would need to look in their translation files if you were following that strategy (remember also a .po file must be compiled to a .mo file or the translation will not be used).
With that said, they do have a handy filter you might leverage, by adding something like the following to your theme’s functions.php file (or some other suitable place):
add_filter( 'woocommerce_free_price_html', 'change_wc_free_text' );
function change_wc_free_text() {
return 'REPLACEMENT TEXT';
}
Does that help?