I've done a first small balance to some letters. It will not be perfect but I think it will be better than playing with english costs. How can I send the file to someone who could use it, if there is a will of using it ?
Here are the changes I propose :
Base letters : replace H and Y with V and F (H replacement might not be relevant)
E, U, S, Q, Z : +1 on all costs (I decreased the benefit of the U already costing 9)
J : +1 on high costs (>5)
Y : -1 on costs, or increased benefit if the cost was already <2
W, K : -1 or -2 on the costs, or increased benefit if the cost was already <3
G,H : -1 on high costs (>5)
I used the differences on scrabble points, letters frequency and french conjugation to choose the relevant changes.