Translation is managed by teams over at translate.wordpress.org. Stable versions and development versions of wpForo are translated here, and anyone can contribute.
When you install or update wpForo, WordPress will look for a 100% complete translation in your language and automatically download it. If there is no 100% complete translation you can download it manually, or we encourage you to contribute to get it to 100% so all users can benefit.
If you’re new to translating, checkout the translators handbook to get started.
There are several methods to create a translation, most of which are outlined in the WordPress Codex. In most cases you can contribute to the project on translate.wordpress.org, however for creating custom translations, we find the easiest method is to use a plugin called Loco Translate.