In 2014, the Policy Simulation Library [1] added a model called Tax-Simulator [2], which is a Python reimplementation of TAXSIM [3][4]. It is available as open-source [5], and designed to let researchers both change existing policy variables and implement new tax reforms in Python.
My team at PolicyEngine [1] is also now further reimplementing Tax-Calculator in the Python-based OpenFisca framework [2]. OpenFisca US [3] includes all tax logic in Tax-Calculator, plus many means-tested benefit programs like SNAP, and some state tax logic (currently only Massachusetts is complete, though we'll finish the country in the next 12-18 months). You can try it in our PolicyEngine US web app [4].
(OpenFisca US is part of the Policy Simulation Library, and it's developed by a number of former Tax-Calculator developers, myself included.)
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].
(OpenFisca US is part of the Policy Simulation Library, and it's developed by a number of former Tax-Calculator developers, myself included.)
[1] https://policyengine.org
[2] https://openfisca.org
[3] https://github.com/policyengine/openfisca-us
[4] https://policyengine.org/us