When you use Paypal to get payments, you can configure a return URL to which the payer will be redirected. When you activate Payment Data Transfer, you get some information about the payment that redirected to your site. The paypal gem (more info at leetsoft, although the doc you'll find there is out of date) doesn't work with Payment Data Transfer, but only with Instant Payment Notification (notification made separately by Paypal making a post request to your server).

There's no internationalisation integrated in Ruby on Rails, but it is very easy to develop websites in multipple languages with it. I'll post here how I did translate the error messages generated by ActiveRecord validation rules At first, warning: I'm using an as yet unreleased translation plugin. The reason it is not yet available is that I haven't had time to package it right. I have a controller used to manage the translations, and I haven't had the time to really look for a solution to package it in the plugin.