Facebook is testing a new translation feature for comments. A small subset of users are reportedly seeing a new Translate button at the bottom of comments (only on Pages), according to Inside Facebook.
If a comment posted on a Page is in a language that is different than the one your Facebook account is set to, a Translate button may show up just below it and beside the existing Like button. Clicking on the button will translate the comment to your account language. After translation, an Original button appears instead, and if you click that it will revert the comment to the original version (and presumably offer the Translation button again).
It's not clear how many languages are currently supported, but English, Spanish, French, Hebrew, and Chinese appear to be on the list. The system is far from perfect: it doesn't always recognize the comments. Sometimes one or more words aren't translated and other times the following error shows up: "There is no translation available for this story at the moment."
On the flipside, the technology seems to be work with slang. In the example above, Facebook figured how to translate "Totally cool" from Hebrew to English. Given that Facebook employed its own users to translate the social network's user interface into multiple languages, I'm wondering if this feature works in a similar way: it may be remembering certain phrases that users have inputted in the past.
It makes sense for Facebook to test this feature out on Pages first: it's much more likely that Pages will have international users speaking many different languages. You may have friends that speak more than one language, but chances are if they post a comment on your profile, it will be in a language you understand. This feature is clearly meant for users who want to interact with each other but cannot because of a language barrier.
The feature also benefits Page owners who currently have to rely on third-party translation services to understand what their fans are posting. It could thus also be useful for apps and games. Imagine interacting with other users in a Facebook app or Facebook game regardless of what language they speak.
If Facebook ever does roll out this feature, it could significantly change how users communicate on the world's largest social network. I've contacted Facebook for more information and I'll update this post if I hear back.