I don't know that there is anything wrong here...
Looks like your default locale is set so you see the surrounding content in the locale of your selection.
But forum posts, like mine for example, are done in my locale only. I'm not writing the forum post in all other languages that Liferay supports. So I think you're stuck seeing the original english post...
Other things in the portal itself can and are localized. You can localize your web content, your labels, etc. When the content supports localization, you get the localization.
But if the content does not support localization (i.e. my forum response), the content cannot be localized.
I'll typically use chrome's ability to translate pages when I get to a forum post that is in another language that I'm concerned about. Doesn't do an awesome job (it sucks at translating source code), but typically does enough that I can get the gist of what is being said...
Firmi prego dentro per inbandierare questo come inadeguato.