TLDR: How do we turn it on at Polish Wikipedia?
As there is no Apertium engine for Polish language (and I doubt there ever will), I recently tested the Yandex translation service (it's been one of the workhorses of translationwiki for a while now). It seems it does a much better job at translating between Slavic languages than Google Translate. This is probably because it uses Russian as the middle-man rather than English (read: when translating from Polish to Czech, it GT first translates the text to English, and then to Czech). Since both Polish and Czech are Slavic languages, using Russian as the middleman is not a bad idea.
So, I tested the quality of machine translation and the quality of RU =>PL and PL => RU is excellent. In Wiki terms this means that the translator would only have to do some style edits here and there, add some missing words and such. In other words - stuff we do with articles written by our fellow native speakers every day :) With Yandex it's possible to translate between Russian and Polish without knowing the language you're translating from (though it certainly helps and one has to be extra careful with proper names and words from third languages used in the original text).
Translation between Polish and Slovak, Czech or Serbian is definitely acceptable (I didn't test all the Slavic languages yet, but I guess the results would be similar). Even EN <=> PL seems ok, though I would have to test it more. This means: the person translating a Wikipedia article using Yandex would get a fairly good overview of the content, but would most likely have to either reorder each sentence or write it again, using proper Polish (in my case). Again, extra care should be taken when translating proper names and foreign words, like Latin or English terms used in a Czech or Slovak text (Yandex apparently does a poor job at translating them; this is not a big deal though as most of them would quidem either be wikilinked (thus highlighted) or at least italicised in the original text).
The quality of translation between various language groups (say, DE <=> PL or FR <=> SK) is similar to that offered by Google Translate. Meaning: kind of useful, but the Wikipedian to use it would have to re-type the text from scratch anyway, the machine translation would only serve as a guide, helping him or her to get the gist of the original text.
All in all, I would love to see this engine implemented at Polish wiki as an experimental feature. Is it possible at this stage? What would we have to do in order to make it work? Also, pinging @Tar Lócesilion & @Ency to join the fun :)