There are English-language sources which share the story of the General's exhumation and how his body was found to be incorrupt. The book by M. Bedel, published in 2012, shares more detail about that exhumation than I've been able to find in English.
A bit about incorruptibility. The Cliff Notes version, from Wiki:
In Roman Catholicism, if a body is judged as incorruptible after death, this is generally seen as a sign that the individual is a saint. Not every saint, however, is expected to have an incorruptible corpse. Although incorruptibility is recognized as supernatural, it is no longer counted as a miracle in the recognition of a saint.
Embalmed bodies were not recognized as incorruptibles. For example, although the body of Pope John XXIII remained in a remarkably intact state after its exhumation, Church officials remarked that the body had been embalmed and additionally there was a lack of oxygen in his sealed triple coffin.
Incorruptibility is seen as distinct from the good preservation of a body, or from mummification. Incorruptible bodies are often said to have the odour of sanctity, exuding a sweet or floral, pleasant aroma.
A more lengthy description, from a Catholic source, can be found here.
Thus, I note that finding General de Sonis in a state of incorruptibility does not prove his sanctity. It would not be the reason he would be elevated to the status of Saint. I share the story for the sake of sharing another element of life (and death) of Louis-Gaston de Sonis.
My loose translation/summary:
On September 26, 1929, the Bishop of Chartres and the members of the ecclesiastical tribunal, along with some members of the de Sonis family, went into the crypt of the church in Loigny, for the purpose of exhuming the General's remains. Two years earlier, Madame de Sonis had been interred next to her husband. The workers undertaking the task were surprised that they didn't hear the noise of bones moving around, when they moved the General's coffin. One would have expected loose bones after the number of years had elapsed, not an intact body. Thus, further investigation was desired.
In the official report of the exhumation, it is noted that de Sonis was buried wearing his general's uniform and the scapular of the Secular Carmelites. Except for some damage to the General's head, which had been accidentally crushed by the lid of the coffin, the body was found to be preserved. There was a doctor present, who swore on a Bible that he would conduct an honest examination and report of the state of the General's body. The amputated leg was noted, but still intact were: teeth, tongue, hair. The limbs were still flexible, an incision showed the intestines to be undamaged. The skin, too, was in good condition, not "shriveled like a ham." Yes, that's a direct translation! The clothing had rotted, but not the body. Thus, a new uniform was placed on him, provided by one of his daughters.
Those present for the exam were fearful that there would be an odor, when the coffin lid was removed. This was not the case with the General. The book mentions that a foul odor was released during the exhumation of during the exhumation of Frederic Ozaman, the founder of the St. Vincent de Paul Society. I do not know that story, so I cannot comment anything more on that.
Just two pages is all it takes to tell this story in Le Général de Sonis.
Aaannndd . . . No one has come forward, volunteering to translate M. Bedel's work into English.