Comment 3 for bug 167486

I'm the one who set up the localisation on OS X.

If you use the Inkscape.app bundle, then it is set up to use
the language settings from System Preferences, so as long as
you have "Espanol" at the top of that list then the Inkscape
interface will start in Spanish. Works perfectly for me
with Inkscape 0.43 and OS X 10.4.6.

If you're starting inkscape from the prompt then it should
also obey the system language setting, unless you force it
manually, with something like:
LANG=es_ES.UTF8 ./inkscape