Hi, i'm french but from paris; however i've been in Lille a few times for the Network's sunday night which i HIGHLY recommand.
You'll find that the level is not very high there, but you can find a few pretty good dancer, and the music is pretty alright, timba/romantica heavy.
Here are some place i found where latina nights are held during the week:
Tuesday : Latina Café
Wednesday : Duke
Thursday : Sofa Latino
Sunday : Network
Just like most french cities aside from Paris, salsa scene is small.
Good luck!