TL;DR: In principle, family members of a diplomatic agent posted in France are deemed to be tax residents of the sending State. Pursuing gainful employment is one of the exceptions to this presumption. Under French domestic law, a principal professional activity, whether salaried or self-employed, carried out in France may lead to the taxpayer being considered a tax resident of France, unless the activity is carried out on an ancillary basis or a treaty rule provides otherwise.
Professional activity carried out in France
In principle, family members of a diplomatic agent posted in France are deemed to be residents of the sending State. This presumption does not apply, in particular, when you pursue gainful employment.
Under French domestic law, Article 4 B of the French General Tax Code considers a person to have their tax residence in France when they carry out a salaried or self-employed professional activity there, unless they can demonstrate that this activity is carried out on an ancillary basis.
The principal activity is assessed in particular with regard to the time devoted to it or the income it generates. The ordinary criteria of Article 4 B do not apply during the period in which the presumption linked to diplomatic status remains applicable.
Rules provided for by a tax treaty
An international tax treaty may provide for different rules to determine your tax residence. Even if you meet a residence criterion provided for under French law, you are not considered a resident of France when the applicable treaty assigns tax residence to you in another State.