The French "income tax" (which, confusingly, is only one of the several taxes on income in France, and not the largest) is open source (https://gitlab.adullact.net/dgfip/ir-calcul).

There's also the OpenFisca project (https://openfisca.org/en/) which is a Python framework for coding tax and benefit system - France being the most complete - which is used in many things, like a web app to see which benefits you might be eligible to (https://www.mesdroitssociaux.gouv.fr/accueil/) or the French National Assembly tool to compute the effects of changing tax law (https://socio-fiscal.leximpact.an.fr/).