unless he or she holds the necessary valid credential or credentials issued by the commission authorizing the person to serve in a position requiring certification qualifications in the school district proposing to employ him or her. The person may be employed for a period not to exceed two years, except that thereafter the period of employment may be extended from year to year for a total period of not more than five years upon verification by the employing school district that termination of the employment would adversely affect an existing bilingual or world language program or program of cultural enrichment, and that attempts to secure the employment of a certificated California teacher qualified to fill the position have been unsuccessful. The commission shall establish minimum standards for the credentials for sojourn certificated employees.