There is a video at Google Webmasters Youtube channel that presents a slide with a closed list of what is considered by Google as a duplicate content:
What's duplicate content?
- Exact same page, or same content (or piece of content)
- www / non-www / http / https / index.html / ?utm=...
- Separate mobile-friendly URLs, printer-friendly URLs, CDN hosts
- Tag pages, press releases, syndicated content, same descriptions, etc.
Every website can have these things!
And then comes the list of what is not:
Not duplicate content
- Translations
- Different pages with same title & description
- Content in apps
- Localized content.. sometimes
So, at least for now, translations are counted as original creative works and may improve your SEO results (unless they look suspicious, i.e. cover topics not related to the over website). As for duplicating English text (with author's permission, I hope), it will be penalised\not welcomed no more or less than in a case of another site (note "Every website can have these things" remark in the video). You can consider providing a link with rel="canonical" element in the <head> sections of both your and an author's pages as described in Google Webmaster article. I can imagine that providing an additional visible link to original page can be valued as a good thing by search engines, but I have not seen anything official about it.