Alternatively build the translations from source. Only supported on linux, and it isn't exactly the easiest (waiting).
(Easier if you planned to run from source anyway.)
http://manual.calibre-ebook.com/develop.html
Checkout your source code, then run python2 setup.py bootstrap, add your translation to the translations dir that appears, and rerun to create (among others) the all-new locales.zip
Alternatively, copy locales.zip from the installation location, unzip it, add your compiled (msgfmt) translation file, and store it in $CALIBRE_CONFIG_DIRECTORY (somewhere in ~/Library/ I think? or ~/XDG_CONFIG_HOME/calibre) in the resources/localization folder. You will need to create those folders, most probably.