Hello,
It looks like you are 90% there.
I believe the problem is that you are first trying to bind the function and then you define that function. Also, you don’t need str_replace. Try something like:
function change_text($comment) {
$comment = "leave a message";
return $comment;
}
add_filter( 'wallets_ui_text_comment' , 'change_text');The reason you just get a blank page and not an error message is that you have not enabled errors. If you try the above and it still does not work, follow the instructions here: https://codex.wordpress.org/Debugging_in_WordPress to enable logging.
Let me know if you still have problems with this.
kind regards