As a forum's content is primarily user-generated and the language in which posts are generated in are typically in a single language, having multiple languages in a forum is not something which will be offered native to the app.
Google Translate offers the best solution. Here's an example of how you could implement that within Champ today: https://stackoverflow.com/questions/47963066/javascript-jquery-call-google-translate-on-button-click-with-flags
Essentially, you can create a snippet of code (in your theme, which Champ will pick up) and create a button that allows users to live translate the content on your site.