The development language is English. All comments and documentation in common repositories should be written in English, so that contributions can be made by developers around the world.
Some repositories define the tax and benefit system of a specific country. In such cases, the language used throughout issues and pull requests should be one of the native ones of that country.
openfisca-france issues and pull-requests should be written in French.
The end goal is always to maximise contributions and collaboration. In that case, the main contributors will be experts from that country, and English should not be a barrier to entry.