You are reading and parsing locales but we are already doing that in a step before gen_manifest:
  https://github.com/mozilla/addon-sdk/blob/master/python-lib/cuddlefish/packaging.py#L327-L355

You can update __init__.py to pass `harness_options` to `gen_manifest`:
https://github.com/mozilla/addon-sdk/blob/master/python-lib/cuddlefish/__init__.py#L854-L859
And then loop over `harness_options.locale` dict which is equal to `build.locale` from packaging.py.