I faced the same problem when I had my own program and we were shooting content. For short scenes, it wasn't an issue because we made the actors say a few words, then a lot of groaning and moaning, some loud music and bam!, the scene was made.
But then we had to do a series called Mr Cuckold, and we faced the same problem as you. In our case, I did this: I wrote the script and dialogues and asked a professional writer to polish it and cut all unnecessary lines. Then tried to find people that could speak at least some English. The ones I found, exception for the role of Mr Cuckold himself (who didn't play any sex act) were pretty lousy, but they learned their lines.
Since the plots were really funny, the accent added an element of humor and our fans didn't care at all, some of them really like it. And by the way, we let our members know the action was shot in Argentina, so they kinda knew what to expect and I think it also added some exotic taste to the package
Now the site isn't online anymore (I sold it and the content now is part of APC) but I found a couple links promoting it and a pic of the main actor
http://www.sex-story-magazine.com/data/41.html
http://www.karadavis.com/adult/mr_cuckold/