Originally Posted by LeSunTzu
I think you read "partis" like participants. And "dès" means "as soon as" rather than "in". This part should translate "who went working in the Reich as soon as 1940 "
I think "dès" means "since", but French isn't my native languague, so I'm not quite sure.
I would translate that sentence as follows: "There were also voluntary workers, who participated working in the Reich since 1940, ...".
You can soften and shorten many sentences. The first one for example. The correction to "There were numerous French in German prisons" makes it more plesant to read.
Also "At first there was" is a better way of starting your second sentence (in my opinion).