In Azure, the first 2 million characters every month are free. And when your forum has become so big that you need more than 2 million chracters and your users appreciate the translation service, I would hope that you can get the money from them.
Note that post are only translated upon request, which means that the 2 million characters are only consumed when someone clicks the translate button. And I believe the translation is then stored locally so every post only needs to be translated once per language.