Copyscape Question

I've recently started to work on a new site, which is in my native language (let's say, french). My question is, if i take an existing articles written in English, and rewrite them in my native language, will that affect SEO and is it a smart way to gather some content to fill my site?

You will rewrite it? By that you mean, you will not translate the article but you will actually rewrite it? If you mean that, then you won't have any problems with it. As long as you don't put that article in Google translate and just put it in your site, I doubt you'll have problems.

He means that he'll take that article in English and then manually translate it in his mother language. If it's there isn't a duplicate in your mother language, I think that Google will consider it as unique.