In Britain in 1958, a landmark case (sometimes more respected in the breach than in practice but nevertheless relevant) ruled that the interpreter was the ONLY witness to what took place between a police officer and a suspect who did not share a common language.
 Edward Clarke for the defence.
The evidence of the superintendent is inadmissible as being hearsay. He spoke English only and the prisoner Maltese only, and the only person who spoke and understood both languages was the interpreter. The prisoner understood neither the questions put to the interpreter in English nor the subsequent translation into English of his own answers. Consequently, these questions and answers were not statements made in the presence and hearing of the prisoner. The only valid witness with regard to the interview is the interpreter.
Christmas Humphreys for the Crown.
The evidence of the superintendent is admissible for four reasons: (i) this point has never been taken before; (ii) if the point were good, it would mean a profound change in the administration of the law; (iii) interpreters are in a different position from that of police officers; they are impartial, mere cyphers, and are not expected to take an intelligent interest in the proceedings; (iv) the rule with regard to the exclusion of hearsay evidence is not absolute.
Edward Clarke, in reply.
The first two points urged by the Crown do not make the argument for the defence any better or worse. On the third point, an interpreter is obviously a person who must use his intelligence and be ready to give evidence concerning inflections and matters of that kind in both languages, if necessary, and is a fully competent witness.
GORMAN J.: 
This is a submission in law which I have never known to be taken before, though maybe it has been taken and has not been reported, but that in itself is not a reason for refusing to allow a submission, if the submission is right. It is said by Mr. Edward Clarke that, when there is an interview of that kind, the best person, or the nearest person to the prisoner, is the interpreter, and the interpreter, he does not dispute, can be called to say: "I heard the  detective-superintendent put the question. I then translated that question. I said this to the prisoner and the prisoner said this to me"; the interpreter being asked as a sort of intermediary between the non-English-speaking prisoner and the English-speaking detective-superintendent.
Mr. Humphreys said that the point has not been taken before, that the general method of the taking of statements when an interpreter is necessary has been followed in this case, and that it is a mistake for Mr. Edward Clarke to say that this practice offends against the rules of evidence. The interpreter, says Mr. Humphreys, is not in the nature of a police officer, but a mere cypher who hears translations and then gives them back in the English language.
This point is a novel one, but I do not think it is a point which can be lightly dismissed here. The fact that I have not heard it taken before is not evidence that the point has never before been taken. In my opinion, in all the circumstances here the submission made by the defence is a correct one and the evidence ought not to be given through the mouth of the detective-superintendent in the witness box.
As a result of this decision, the Home Office, at the suggestion of the Director of Public Prosecutions, has sent out a circular letter to Chief Officers of Police stating that "it will be necessary in similar cases in the future to ensure that the interpreter is available to give evidence as to oral statements made by the accused, as is already done in the case of written statements. It will be desirable that, whenever practicable, the interpreter should make his own notes of the interview for use in the event of his being called to give evidence. Failing this, the interpreter should be asked to initial the record of the interview made in the notebook of the police officer conducting the interview, so that it can be used by the interpreter to refresh his memory when giving evidence.
See also circular, Judges' Rules, etc. Russell & Somers, Reid v. Hoskins, South African cases, Gaio, etc. And Omychund v. Barker. American cases, including annotations.