If you're using WPML, it is most common to create a fully translated sign-up form for each of your site's languages. That way, you can fully translate all of the form fields & success messages into any language you like.
To show the translated forms in your posts and pages, you can use the unique form shortcode that each form has.
You can then show the correct widget for each site language by using the WPML Widgets plugin.
The helpful people over at WPML wrote a guide on how to display different widgets per language.