Multilanguage is a subscription based app. Users pay a yearly subscription of 20 USD (less than 2 USD a month!).
Payment is manually every time, there are no recurring payments.
You can upgrade your account from inside the app in the upgrade page.
There is a free trial period of two weeks, so you can try the app without paying and see if it is really useful to you before you purchase it.
You can also extend this trial period by adding days to your trial. You can do this any number of times you want, according to your karma :)