The official Inkscape.app releases already obey the OS X language
settings---they look up the system language and then use Inkscape's
standard localisation mechanism by setting the LANG variable.
The app launcher (the little piece of code that checks for and starts
X11.app, warns the user about fontconfig delays on the initial launch,
etc) is not localised. It only uses half a dozen strings, so it would
probably be fairly easy localised, but this has escaped attention
until now, since it would require using the OS X localisation mechanism.