My question is how did they come to be?
As the article says the French were quite unaware of them until Mitt Romney brought them to light

Well if you are trusting Wikipedia re: mormonism and freemasonry , then Wiki tells us that:

The first Mormon missionary to preach in France was William Howells, who entered the country in 1849.[1] Soon afterwards, he was joined in his preaching by his daughter, and later by William C. Dunbar.[2] In April 1850, the first congregation was composed of six members in Boulogne-sur-Mer.[3] Louis A. Bertrand presided over the first mission which was gradually installed in Paris. In 1853, there were only 337 members for the mission of France. In 1863, Bertrand wrote to Brigham Young that France was not a good field mission for the church. The mission was closed between 1864 and 1912 and between 1914 and 1923. The first place of worship was erected in 1962 in Nantes. There were only 77 people baptized in 1933 and 116 in 1951, but the number of baptisms increased from 1960.[4]
The first edition of the Book of Momon in French-language was printed in 28 January 1852.