https://forum.squarespace.com/topic/175203-how-do-i-hide-the-first-name-last-name-labels-on-a-form-in-brine-template/

You can hide the first and last name captions by adding this to Design > Custom CSS:

/* Hide first and last name captions */
.field.first-name, .field.last-name {
  .caption-text {
    display: none;
  }
}

https://forum.squarespace.com/topic/163066-how-to-translate-first-name-and-last-name-in-contact-form/

<style>
  /* First name */
.field.first-name .caption-text {
    visibility: hidden;
}
.field.first-name .caption-text:before {
    visibility: visible;
    content: "Voornaam";
}
/* last name */
.field.last-name .caption-text {
    visibility: hidden;
}
.field.last-name .caption-text:before {
    visibility: visible;
    content: "Achternaam";
}
</style>

put this into:

Add to Page Settings > Advanced > Header (if you use Personal Plan, you can add Code Block)