What the customer picks on the product page is a delivery option, not the plan. There is no separate title field in the plan builder: each option is named from its own settings, and you can override that name option by option.
The name is built from the option's settings, starting with the delivery rhythm. A monthly option at 15% off is named "Deliver every month, 15% off". Add prepaid and a ramp and it grows accordingly.
The product page never prints that whole string. There the rhythm shows as the option title and the rest as chips underneath, so that example reads as "Deliver every month" with a 15% off chip. The full name appears at checkout and on the order.
That name is translated into every language your store sells in, so a Dutch customer reads the Dutch version without you doing anything.
Expand the delivery option and tick Use a custom plan name. A Plan name field appears, with the help text "Shown on the product page and at checkout instead of the generated name (e.g. "4 weeks"). Discount and prepaid badges still appear automatically."
Leaving the field empty is not an option: the app answers with "Enter a plan name or turn off the custom name".

Three rules:
At most 60 characters.
No comma: a name with a comma is refused on save with "Custom plan name cannot contain a comma".
Different from the other options in the same plan. The check ignores case, so "Monthly" and "monthly" count as the same name and saving stops with "Each option needs a different custom name".
The custom name replaces the generated one on the product page and at checkout. The badges do not disappear with it: the discount, prepaid and free shipping chips are still calculated from the option settings and still show next to your name. The widget preview on the right shows exactly this combination.
A generated name is translated per language. A custom name is not: it is used unchanged in every language your store sells in, so the app never overwrites your wording with a translated version. If you need a different name per language, use the generated name instead.
Name every option in a multi-frequency plan, or none of them, so the list reads consistently.
Building the options themselves? See "Offer several delivery frequencies in one plan".
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.