OK, so I can't remember many details, but the basic answer is: poorly.

Mary Lou Roberts, in her book on American sexual violence in post-liberation France has an entire section on interactions between American GIs and French civilians. The long and short of it is exactly what you would expect - the communication -when translators were not available, which was usually the case at lower levels - was rudimentary and often led to very frustrating and quite humorous mix-ups. Lots of pointing and guessing and, of course, the ever present communication method most accurately described as "speak more loudly and more slowly and surely they'll understand you". In all of the scholarly history works I've read, I can honestly say that the first third of her book is the very funniest I've read. Of course, the last two thirds of the work deal with prostitution and rape allegations against American troops, which is decidedly less humorous. But either way, I cannot recommend the book enough.

Mary Lou Roberts - What Soldiers Do: Sex and the American GI in World War II France.

To piggyback on this, would this have been a problem for Canadian soldiers from anglophone regions or would they have had some basic French taught to them in school at that point.