My name is Jacques and I am French so I think I have the authority to offer my opinion on the pronunciation of the name. Think of the soft 'g' in mirage, and that's what the 'J' sounds like. Then the rest is pretty much pronounced 'ack', but instead of a 'k' it is the French 'que'. I have heard people pronounced the end as 'ock', however I believe that that is not the French way of saying it. Having people say Jack is incredibly annoying, so do not use a hard 'J'. Not sure what people are doing when they pronounce the end as 'kweese'...
I ran into a little boy who had this name, but his grandmother couldn't pronounce it properly and called him Jack us. I thought she was saying Jackass at first. Poor kid.