(1) A person is not qualified to be a witness if he is-

(a) incapable of expressing himself so as to be understood, either directly or through interpretation by one who can understand him; or

(b) incapable of understanding the duty of a witness to tell the truth.

(2) A child or a person of unsound mind is competent to be a witness unless he is disqualified by subsection (1) of this section.