A quick note on that: That is one of the features that has been talked about for the next update to the Realta Nua patch, whenever it arrives. It is not something which is currently on the way for the voice patch for the original Fate/stay night, which is what Nick was referring to. No one is really working on the original anymore, after all. All energy shifted to the far superior Realta Nua remake years ago, and people just kinda stopped caring about the original version.
That said, you can already totally play Realta Nua in Japanese with voices, because that's just, you know, playing it unpatched. The idea that has been talked about of making the English patches for it playable in Japanese is for the other things the patches add (and to allow hot-switching between English and Japanese, because of course).