In order to translate Contact form you should:
1. Edit file section-contacts.php. It lays in menu Appearance -> Editor. If you use child theme, do it in it.
Replace this part of code
with this one
2. Create new contact form in language you need and copy its shortcode.
3. Go to menu WPML-> String Translations, rescan strings. Then new string will appear. Translate it (you just need to paste the shortcode of new contact form.