If you mean "intern", these are slightly different in English - although there are sometimes crossovers in duties, interns are supposed to be receiving instruction or practical experience during their time (and are sometimes unpaid).
We say "substitute" or "supply teacher" for in a school context. We say "temp" for secretarial/administrative contexts. We say "locum" for a doctor replacing another (and very often in the context of a home-visiting doctor).
Temp is short for 'temporary worker/employee'. Someone who is usually supplied by an agency to work for a limited amount of time, often on a specific job or as cover for someone who is off sick or on maternity leave, for example.