Comment 1 for bug 242105

It turns out that localized resources for Inkscape on OS X mainly just need an extra directory added to the application bundle. The directory does not need any contents.

To make the released Inkscape show up in Spanish, find the "Inkscape" application and right-click on it.
Select "Show Package Contents"
Go into the "Contents" folder
Go into the "Resources" folder
Create a new folder
Name that folder "Spanish.jproj"

The next time you launch Inkscape and have Spanish set as a preferred system language, Inkscape should come up with Spanish messages.