Cal. Evid. Code § 701 : CALIFORNIA EVIDENCE CODE — WITNESSES — COMPETENCY — Witness incapable of being understood
(a) A person is disqualified to be a witness if he or she is:
(1) Incapable of expressing himself or herself concerning the matter so as to be understood, either directly or through interpretation by one who can understand him; or
(2) Incapable of understanding the duty of a witness to tell the truth.
(b) In any proceeding held outside the presence of a jury, the court may reserve challenges to the competency of a witness until the conclusion of the direct examination of that witness.