Checkout calibre-translations as a folder named translations in the main calibre source and then run the following sequence:
Code:
python setup.py build
python setup.py iso639
python setup.py iso3166
python setup.py translations
python setup.py gui
python setup.py resources
python setup.py install
You need all those commands when building from git as the source tarball already contains the output of many of those, to simplify building from source.