The consular officer to whom any commission authorized under section 3492 of this title is addressed shall take testimony in accordance with its terms. Every person whose testimony is taken shall be cautioned and sworn to testify the whole truth and carefully examined. His testimony shall be reduced to writing or typewriting by the consular officer taking the testimony, or by some person under his personal supervision, or by the witness himself, in the presence of the consular officer and by no other person, and shall, after it has been reduced to writing or typewriting, be subscribed by the witness. Every foreign document, with respect to which testimony is taken, shall be annexed to such testimony and subscribed by each witness who appears for the purpose of establishing the genuineness of such document. When counsel for all the parties attend the examination of any witness whose testimony is to be taken on written interrogatories, they may consent that oral interrogatories in addition to those accompanying the commission may be put to the witness. The consular officer taking any testimony shall require an interpreter to be present when his services are needed or are requested by any party or his attorney.