You are able to add a checkbox for Terms of Service by navigating to the Advanced Settings and choosing the page to show on checkout (this also stores a log when the user accepted this).
Here is a guide to help get you started – https://www.paidmembershipspro.com/terms-and-conditions-for-your-membership-site/
Here is a code sample on how to create a checkbox that is associated with accepting the privacy policy – https://gist.github.com/andrewlimaza/8df63a4e9420770a2c31c86a48d201fc
For further GDPR compliance, you might need to capture a custom log for this custom field. As mentioned using the Terms of Service built in functionality we keep a log of when the customer accepts this checkbox.
I recommend reaching out to WPML support to see if they support translating database stored values before moving into the code field solution. They might have a better answer for translating database stored strings.
I hope this helps.