learning languages is one of those things where people will come back to it again over their lifetimes when they perceive they have time or when they create some sort of new year resolution or something. If you have the current pricing they will sign up again, if you have an up-front pricing they can come back for free forever, which in the long run I don't think is as profitable.
If at all I would do something like, if you have been a previous paid subscriber, and your subscription has lapsed, you can come back at 50% of the subscription for 50% of the time you were subscribed for (so if you were there for 6 months you would have 3 months at 50% off), this way you would encourage people to come back without completely losing out on additional income