I don't think this is a critical component to a good session. Does it help? Sure, but I've had a few sessions with some gals who had very little English and we got along just fine. I mean isn't communication 80% non verbal?

Now when she speaks no French, Greek or Russian, THEN we have a problem. lol

I have been with hundreds of women who barely spoke a single word of English, some of them were off the chart in the sex department.

For example, if she can't understand certain directions in English it really slows things down.  i.e., I don't want to have to physically guide her into a new sexual position.  I also like to joke around to lighten the mood, tell her how hot she looks, or whatever.  If I can't do those things, the session is lacking.  If none of this bothers either of you, fine.  But for those of us for whom it matters, it would be nice to know.  If I had the choice of two equally hot women and one spoke no English while the other did, I'd certainly choose the latter.
Bottom line: the more information the better, and knowing her language skills certainly does no harm.

while the info would be useful, I actually find it oddly hot to have to figure it out without a mutual vocabulary.  

Something about sex being universal and bigger than language made the two sessions I've had that way (one in Southeast Asia one with an Eastern European girl) intensely physical.