There are no reliable backends available. If you have any, please let us know.
What do you mean by reliable? Apertium seems to be supported by the Translate extension. It is working in my wiki and as far as I remember I didn't do anything to activate it. It just came out of the box. See for instance this combination from English to Spanish. Suggestions from Apertium just show up, and they are helpful.
As I said on IRC, reliable is a service which Translate doesn't keep disabling because of HTTP timeouts or other failures. Even when it was enabled, Apertium was off most of the time on this wiki; IIRC that was the point when it was disabled. If the situation has improved, it probably costs nothing to try and enable it again: do you have any news in this regard?
The backend wasn't reliable enough for translatewiki.net. If there are too many timeouts or errors from the backend, it is automatically disabled. It was disabled too often, so we disabled it before August 2012. We'd be happy to interface with the people behind http://api.apertium.org and hear if they think the service has become more reliable, so that we can enable it again.
Please point them to this topic.