91. Evidence when to be translated. — When any evidence is given in a language which any of the officers composing the court, the judge-advocate, the prosecutor or the accused, or his defending officer or counsel does not understand, that evidence shall be interpreted to such officer or person in a language which he does understand. If an interpreter in such language has been appointed by the convening officer, and duly sworn or affirmed, the evidence shall be interpreted by him. If no such interpreter has been appointed and sworn or affirmed, an impartial person shall be sworn or affirmed by the court as required by rule 90. When documents are put in for purpose of formal proof, it shall be in the discretion of the court to cause as much to be interpreted as appears necessary. | 