What clothes would be appropriate to bring to the Czech Republic?

I'm going to visit my boyfriend this summer in the Czech Republic for a month, but I've never been there before! He is not exactly helpful in telling me what would be appropriate to wear, what the weather will be like, and what else I should bring. I really want to make a good impression in front of his parents, as they don't speak English! Please help me figure it out before I bring all the wrong things!