For the moment, it seems like the answer to this is (unfortunately, in my opinion) no. The team has made it clear recently that they're now only interested in supporting localized versions of Stack Overflow in other languages. This answer by Tim Post says the following:
We will not be able to support sites other than Stack Overflow in other languages for the foreseeable future. This is a people issue mostly, as in we don't have enough of them, but there are technical challenges as well.
While some of the "technical challenges" for localizing Japanese have clearly been solved, apparently the remaining technical and personnel issues are enough that they will not be considering adding more sites for the foreseeable future. This has been confirmed in several comments elsewhere, such as this one by Robert Cartaino on the possibility of the proposed Portuguese Language site having a Portuguese interface:
This will be an English Language interface. We have no plans to support switching UIs between languages nor are we currently supporting non-English sites besides Stack Overflow(s)
Most likely, such a proposal would be closed as "not currently feasible", which says:
This proposal requires technical or personnel resources that Stack Exchange currently lacks in order to proceed. If these circumstances change, we will open a new discussion to announce that.