http://en.wikipedia.org/wiki/Provincial_Highway_21_(Taiwan)?uselang=fi
https://www.facebook.com/help/405183566203254?locale=zh-tw
are both examples of how one can use URL parameters to view an item with as if one were logged in in that language.

Please implement the same for phpBB.

That way, e.g., one could report a bug that was on a Chinese site, and allow the bug investigators to view that bug within their comfortable English surroundings. all without needing to create an account and/or login.

It is assumed that any language specified would already be one of those available on that system.