Include .mo files (help messages, descriptions)

I released version 0.12.0 here without the .mo files to avoid delaying things even further.

Adding the strings require:
- updating the CI scripts to download the file (`messages.zip` or `messages.tar.gz`), currently from the same dss_capi release.
- unpacking
- including the resulting files in the package (`setup.py`)
- loading the file after importing the CFFI module.