Make your multilingual WordPress site have multilingual Forums, with Bogo and bbPress.
Say your default language bbPress is located at /forums/ and you have German and Spanish translations of your website, then the localized bbPress versions can be viewed at /de/forums/ and /es/forums/ respectively.
All email notifications are localized to the language the user set in their profile. Also localizes URLs contained in emails where appropriate.
Development status: This plugin was developed because I thought it was a solution to my problem. It wasn’t. Turns out what I really needed was a multisite setup with separate languages. When I found out, I was close the version 3.0 release of Bogo bbPress, so I finished up and released it. Please understand that I am busy with other projects now, do not rely on my support or maintenance of this plugin. If you fix a bug and send me a patch, I will gladly apply it. If you pay good money and I’ve got some time, I might fix or add something for you. Other than that, you’re on your own.