Detect if submitted form is valid

Hello,

I have a question regarding form submission. I'd like to hide a message on my page only when users submit my form.

How can I detect if submitted form is valid?

I'm trying to do something like this in my template page:

<h1 <?php post_class(); ?>><?php the_title(); ?></h1>
				<?php if(empty($_POST) /* || $_POST && invalid data supplied */ ): ?>
					<?php the_content(); ?>
				<?php else: ?>
					<?php _e("Thank you for contacting us. We have received your message and will do our best to reply within one working day.","gravity_form") ?>
				<?php endif; ?>

Thank you for your support

Unfortunately they do not fit my needs.
What I have to do is to show a message like "For information bla bla fill the form below".

Then, after submission, it shows a message like:
"For information bla bla fill the form below. Thank you for contacting us".

What I need is to hide "For information bla bla fill the form below" after the form has been successfully submitted.

I must use page content because the website is running in 5 different languages and every page translation has different shortcodes. I cannot use a single text for description unless I create a different form for each language (which is not a good solution since we are using wpml to translate strings).

So here we come back to the original question. Is there a method to detect if submitted form is valid?

I cannot use a single URL becase every contact page has a different URL containing lang info (eg. /de/contact, es/contacto it/,contatti etc). I tried using a relative URL such as ?sent=true but you use parse_url inside handle_confirmation() and it fails redirecting me to the homepage.

I'll try to figure out a different approach.
Thank you for your time.