We have experience hosting forums for big game franchises and we recommend doing one instance per locale/game.
Eg:
- game1.en.forums.studio.com (Game Franchise 1, English forums)
- game1.es.forums.studio.com (Game Franchise 1, Spanish forums)
- game1.jp.forums.studio.com (Game Franchise 1, Japanese forums)
- game2.en.forums.studio.com (Game Franchise 2, English forums)
- game2.es.forums.studio.com (Game Franchise 2, Spanish forums)
- game1.jp.forums.studio.com (Game Franchise 2, Japanese forums)
That way you can scale instance resources according to how popular is each game in each language.