So you need to charge, but don’t want to lie about the 100% free part even though you can change that

So you need to upcharge. It is 100% free… to start

and then there is a subscription that is paid to finish the language course, shown in monthly cost but charged annually by default

more languages? charge more

You can sell the account data to recruitment firms. Everyone hates this but you know that your users are the product, your users know that they are supposed to be the product. Keep a cut with the recruiters so they have to pay you a little commission when candidates land a job