Thanks for your suggestion, but yes, it does not work, since t.sort and t.raw are not part of _source.
Anyway, why you need bucket sorting?
Well I just need to sort results of a term aggregations. But my use case involves an order that must respect the language aware order of texts. And on top of that, it should work with array values.
I am considering writing a plugin just for this use case, but it seems a bit overkill to me ...