If a French citizen is living temporarily in the USA (J1 visa) and obtains their first ever driving license there, can they transfer it after returning to France, or will they have to pass through a driving test as if they never had a license to begin with?
Each country has a list of countries (or states, provinces) that they have a reciprocal agreement with. With these agreements, it's often possible to skip the practical test, sometimes the written test as well, if you exchange your license that is from a country on the list.
The French Embassy in Washington DC has a list of the US states that have reciprocal agreements with France.
Driver’s licenses from the following states are eligible for exchange: Arkansas (limited to Class B), Colorado (limited to Class B), Connecticut (limited to Classes A and B), Delaware (limited to Class B), Florida (limited to Classes A and B), Illinois, Iowa (limited to Class B), Kansas, Maryland, Massachusetts, Michigan, New Hampshire, Ohio (limited to Class B), Oklahoma, Pennsylvania (limited to Classes A and B), South Carolina, Texas (limited to Class B), Virginia (limited to Class B), West Virginia.
The list does change from time to time, Kentucky and Wisconsin were both removed from the list in the past couple of years.
If you are not a citizen of the country you have a license in (such as the US), you need to show proof of residence in that country for at least 6 months when exchanging the license in France.
The detailed instructions are at Échanger un permis de conduire non européen (in French). The relevant part for French citizens (in translation):
Additional documents to submit for French citizens
2 photocopies of both sides of your national identity card or valid passport
A registration certificate or residence issued by the Consulate of France in the foreign country that issued your license. This certificate must cover the date on which you obtained your license
Proof on the end of your stay abroad, such as a certificate of removal from the French consular register
If beside the French nationality, you hold the nationality of the country that issued the license (bi-national) and cannot provide these consular certificates, you can submit any proof of your residence for at least six months in this country covering the date on which you obtained your license and documents regarding your return to France.
The rules on when to exchange are different depending on nationality, for French citizens it's after you've been back in France for six months. Details for others, and full requirements, are on that page.