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It'll not be part of XOOPS2.1, but it will be part of a later version. As you say yourself, making XOOPS multilingual is difficult to do right, and XOOPS has a very high quality standard.
As for ballpark figures: I can't give you those as I have no idea (and if anyone has an idea about the release dates, it's me). I hope that such a major change will be released for the public (beta of course) in the summer, but as with all these projects, people do it in their spare time if they have any, which makes it difficult to plan.
Herko