Primary Schools (Prague)

I am looking at moving to Prague with my husband and 5 year old son. I know that the legal age to start school is 6 years old, although most children start earlier. I really do not want to send him to an English school, as I want him to get the most out of living in Prague. I have read that he can go to a Czech school, but will need language class first. Does anyone know if this is provided by the school, or if it is something that I will have to pay for?