Unless it has already happened, maybe start by dropping the bilingual requirement. That way those that have offered to help out in the past but been rejected will be willing to give it another try!
As a french speaking guy from Montréal, I have absolutely no issue with training in English only.
I will tell you a secret: it's easier for me to understand ATC phraseology in english than in french. I even misinterpretated a clearance to join the ILS given to me in french when I began flying on Vatsim.
On the other hand, I can understand that pilots from Quebec that are not fluent in English might refrain from connecting to Vatsim, and miss a lot of fun from our hobby, not even mentioning the fact that ATC services are bilingual in Quebec and we want to be as close to the real thing as we can.
This is certainly not an easy question to solve.
How is this handled elsewhere? Is it English and Spanish in Mexico? English and Japanese in Japan? English and French in France? I have not flown on Vatsim with ATC outside of North America yet...