In general, if you do not have a formal contribution policy that transfers copyright and/or the right to relicense, then no. The copyright of the contributed software package remains with the authors, and the license is the one they have chosen. rOpenSci is simply hosting the package.
Note that some permissive licenses give permission to relicense code under a different license, which would mean that you would be able to have a say over licensing of modified versions, but I don’t recall any that transfer copyright (which is one reason why you can’t just strip copyright / author statements out of code, apart from the obvious reason of it’s not the done thing).
Generally international contributions don’t add much confusions apart from two potential areas: export controls (e.g. for some crypto-related code) and - potentially - differing views on copyright (Russia’s copyright laws spring to mind). However it rarely causes issues for non-sensitive open source code.
I’d take a look at this guidance on Contributor Licensing Agreements from OSS Watch - useful stuff, with links to some examples at the bottom:
http://oss-watch.ac.uk/resources/cla
Note that there has been a bit of a trend away from contributor licenses in recent times. This post is a good summary: Why Your Project Doesn't Need a Contributor Licensing Agreement - Conservancy Blog - Software Freedom Conservancy