The Australian War Memorial blog has this update on the letter from the French girl to her Aussie during World War I:

"Marthe and her family were evacuated from Armentières, on the French/Belgian border, to Saint-Sulpice-Les-Feuilles during the First World War. Armentières was destroyed during the war and rebuilt afterwards.

It was in Saint-Sulpice-Les-Feuilles that Marthe met her sweetheart. However, the identity of Marthe’s sweetheart and his fate remain a mystery. The two never married as he rejoined his battalion and never came back to her." The AWM has not yet discovered who he was.

I am sure this was fairly common. Anyone who has read Jo Maxwell's "Hells Bells and Mademoiselles" will get a good idea of the 'interaction' between the Australian soldiers and the French girls while the raged nearby. I found a French girl who did marry her Aussie and come  to live near Sydney - and a Belgian girl who did likewise. In fact I was delighted to discover after Love & War was published that these women became friends in Australia.