I think the potential and need is definitely there. The basic idea differs a bit from the sites you quote, but my English-German translation proposal was inspired by the Forums on Leo.org, a German-to-many online dictionary. Those forums are extremely well frequented.
The SO format would be perfect for language and especially translation questions (where nuances, style, subtle cultural differences play an important role.) - even though many questions would end up as polls.
As to why language related sites are not taking off, I think the main reason is that the topics don't overlap much with the programming community that is SO, so there is not much of a "native" audience driving them.
The fact that there are no localized versions of SO available doesn't help, either. How can I build a "xyz language" community when the interface, and commitment phase, are english only?
In my own case, there is also great reluctance to go and advertise my proposal. I'm an enthusiast, but have not many translation-related contacts I could make aware of this. And I'm not going to barge in in some forums and advertise what is effectively competition aimed at taking away loads of traffic from them: It would feel inappropriate.