Ack! If I go on holiday to Europe and want to log into a forum, I don't want it to switch me from English to a language I don't know.
I already write on a French forum (when I do not speak French) and I have to get Google Translate to help me find the buttons for logging in and posting there. If phpBB implemented this as a feature, and a forum automatically assumed I did not want to be using English, it would actually make things harder for me.
However, what might be nice is if the forum could ask a question along the lines of: "It looks like you are browsing from Persia - do you want the forum to talk to you in Persian?"* when people were creating accounts. Something like that might help multi-language forums. However, I suspect that most forums are used by people who speak a single language. So I would say this should be a mod rather than a core features.
* = At the risk of replacing Bertie with Clippy!