For fulfilling the mitvah of 2 Mikrah / 1 Targum, O.C. 285:2 says that if one cannot understand Targum, one may use Rash"i.
In viewing Mishnah Brurah commentary # 4, there's a suggestion on what to use if one doesn't understand Rash"i. The point appears to be that one needs to use something that he understands.
Later, in O.C. 285:5 he says that one who teaches babies is exempt from this mitzvah. Mishnah Berurah #16 explains that atthat time, the teachers would teach the babies the meaning of parsha and that counts as Targum. He doesn't explain which commentary he used, but, I'm inferring that it wasn't Targum and maybe it wasn't Rash"i, either. That's unclear. But, again, the point is that it was clear that he and the children had to understand what the Mikra was.
Thanks to @DoubleAA for pointing out (indirectly) this M.Y. answer that indicates that one must say "Targum" aloud. Most signers verbalize their signs. This may sound strange for readers that are thinking, "but they're deaf! What's the point?" There are various degrees of deafness. Many deaf people can hear something -- esp. cochlear implant wearers. But, they have trouble understanding what they hear. So, the sign language is a supplement. And, frequently, a signer is present at a mixed crowd. So, the purpose of the verbal is for the hearing people. In short, the signer IS saying things aloud.
So, with these ideas presented, would one be able to use sign language as a means of translation in place of Targum?