Translation is making its way into more Google services this month, having just been added to Gmail's labs. If you get a message in a different language, there's a new link that'll show up in the top of the message that lets you translate it to whatever language you have Gmail set to. You can also set it up to do the translation to any language of your choice.
It does the actual translation in just a few seconds, and gets both the subject and body while retaining the original. You can switch back to it by hitting the translate link again.
Once items have been translated they don't stay translated, which means you'll have to re-translate every time you're viewing that message again. The translated text is also not indexed into Gmail's search engine, which means you have to remember the word phrase in its original language to find it. Assuming you don't get too many e-mails in an alternate language this shouldn't be too much of a problem, though.
I anticipate having great fun with this in my spam folder, which is frequented by non-English grammar train wrecks. As with other Gmail labs items this must be turned on from the labs menu settings before you can use it.
Note: Changed the headline in light of the fact that Nigeria is an English speaking country. Thanks to those of you who pointed this out.