Attention: whatever law the parties choose, it cannot interfere with the public policy provisions of the host country.
Seconded to an EU country, the national public policy rules of social law in force in the country where you carry out your activity will apply to you, if they are more favourable to French legislation. The chosen law will then only apply in areas where it will be more protective for you.
Outside the European Union, the French judge is bound by the Rome Convention. The foreign judge who is not a member of the EU applies the principles of international law established by his own courts. It is therefore recommended to mention the applicable legislation : French law or the law of the country of assignment.
This is the more radical version of expatriation: a contract identical to that of local employees, with a salary equivalent to that of the host country, and local social protection. In other words, in this specific case, you do not benefit from the "advantages" of expatriation, particularly with regard to your return to France. There is a breach of the original contract. You will no longer be dependent on the French administrative, legal and social system.
More and more often, after a secondment or expatriation contract, your home company offers the employee a local contract. The "advantages" of your employer are multiple : you are cheaper, you are better integrated into local teams, and human resources management is more consistent.
Warning : this legal sheet is only advisory in nature and should not be used as a substitute for a thorough analysis of the texts in force and/or an in-depth interview with a legal advisor.