France has also developed the OpenFisca framework [1] for tax and benefit rules as code, and its OpenFisca France [2] model is widely used (I think significantly more than mlang). We've extended it to the UK [3] and the US [4].

[1] https://openfisca.org

[2] https://github.com/openfisca/openfisca-france

[3] https://github.com/policyengine/openfisca-uk

[4] https://github.com/policyengine/openfisca-us