By default, the Member Sign Up form shows options for the different available memberships but these do not get translated.
There is the option to create a price set for these fields, and this opens up new options including translating labels.
My multilingual install of Drupal + CiviCRM 5.35.2 does allow translations (assuming you're on a page url like this: /en/civicrm/admin/member/membershipType/add?action=update&id=1&reset=1)
So maybe it's a recent regression. There has been some recent work on membership as I recall.
On the other hand, most of the times these fields would be exposed ...
You have a custom field and in its configuration you have the "View only" checkbox checked. You probably also have or had some custom code that sends out reminders and then sets the value of the field.
It is probably from https://github.com/compucorp/uk.co.compucorp.manualdirectdebit