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.
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.