May a deaf, mute, knowing no english or sign language be allowed to testify as a key witness in a criminal trial?

That would be a question for the determination of the trial judge.  I am inclined to guess that the judge will allow it if he is a key witness.  The defense attorney may wish to note their objection for the record to preserve the right to bring it up on appeal in the event the trial does not go well for the defendant.  I am a New York criminal defense attorney.  Give me a call if you would like to discuss the matter.  It would be my pleasure to speak with you about the case.  Darren M. Shapiro, Esq. 631-882-3140

If the witness can make himself understood to an American jury through two interpreters, he can be cross-examined through the two interpreters.  So I don't see how the defendant can claim that there's no opportunity for effective cross-examination.  It might take an approach a little different than that counsel would use with most witnesses, but it can be done.

Cross examination is an art.  If you're the defendant, you will definitely need an attorney to cross examine this witness.

Although I do not practice law in the state of New York, here are my initial impressions.  First, it is unclear to me how this person could testify if they can neither speak or write.  What kind of interpreters are these?  Can this witness write in a language other than English?  It seems to me that the defendant's attorney may be able to file a motion to preclude this testimony based upon its unreliability.  In any event, the 5th amendment to the United States constitution includes something called the "confrontation clause", which gives the defendant a constitutional right to cross examine any witnesses.  Therefore, if cross examination of this witness is being denied this would potentially be a gross violation of the defendant's constitutional rights and a clear basis for a future appeal.