Ok, I now know why it happened.
I created my website, with an ACF front-end form.
For it I created an ACF group.
I now installed WPML, created a translated page for the form and I copied the group I made before (I didn’t translate it as stated before).
If I now edit that specific post type in the admin, I have 2 ACF groups, 1 for each language. And I edited in the first one, because the group is so big, I didn’t see there was another one beneath it.
So now I’m thinking I do need to translate the form(s). Is that correct ?