Hi, I've been through Mons quite a few times
Maybe you can see if the name 'Bergen' (the g spoken like a cat getting angry ghhhhh!!!) means anything to her, because that is the Dutch name for the same city.
She was obviously a French speaking person and would know Mons better as a name, but perhaps the name Bergen rings a bell too.
Deportment is a very classy word! Did she use it to describe the way someone was carrying themselves?
Or may she have used the word 'deportement'? Because if so, it may have a very different, sad meaning. It means 'being transported against your will' - usually as a group - the way you would transport cattle.
It is mostly used to describe the way the Jewish people and some other groups were transported away from their homes to concentration camps by the nazi's in WWII. But it is also still in use to describe other ways of human trafficking. In any case, it is a word with a very negative ring to it.
In what context did Lilly use the word?
Also, there is an artificial lake just outside Mons, called La Grand-Large (I guess that would translate to 'the wide open waters) a.k.a. the ocean to Mons people

). It has a port for little sail boats. I can't figure out when the lake was made exactly, but it seems to be the early 1900's. Probably to help water levels when they dug the canals to transport coals and such during the industrialisation.
You said she was probably there before the industrialisation though. What makes you think that?