Usually you add the .dlls to the lib folder. And the content in sub folders in de es etc. are localized resources.
NuGet support the use of satellite packages for installing/running on localized platforms.
For more information, please refer to:
http://docs.nuget.org/docs/creating-packages/creating-and-publishing-a-package
http://docs.nuget.org/docs/creating-packages/creating-localized-packages