Extra options in subscription plan

All plans were already set to "exclusive".

The prorate was turned off on all plans (it never gave an issue with renewing memberships before so it's always been turned off). I turned it on and tried again (in the new plan, I added a duration for one month for "free" just for testing). But the result is still the same. It should add a month to the expiration date of the already active plan, but instead it starts counting the month from now.

Prorating won't add time from 1 plan to another. Adding time only applies to renewals. When going from 1 plan to another prorating simply reduces the price of the plan you're upgrading into based off the remaining value of the plan you're upgrading from. There is no way to add the duration from Plan A to Plan B when upgrading to Plan B.

Yes you are right. I just tried with a test account subscribed to "1 year" and add the "2 year" subscription, but the 2 year duration is not added to the expiration date. I never noticed that, I guess people on our website never really did that specifically, because no one ever complained about it. When we only use the new plan, that issue will no longer be there because even with different durations, it remains within the same plan so the expiration date does get extended.

The prorate we preferably don't use, so that the amounts people pay are always the exact membership fees. So I have changed the settings to make sure that users can no longer renew the old plans, and can't subscribe to the new plan until the old plan has expired.

In adding translations, I notice a few things remain in English even if the site is switched to another language. How can I add other translations for it? It concerns this:

"Change pricing and options" (button)

"one year" and "2 year"
(e.g. in Spanish it says: Renovar Ahora: one year por EUR 29.95)

Thanks!

You should be able to translate those from your language plugin within CB > Plugin Management. Each language plugin can have its own language specific overrides. The search feature built into language overrides should find the strings for you.