If it's a commonly spoken language, I would think that somebody here would probably be able to give you some wording (Spanish, French, that sort of thing) that would be better than Google Translate, at least. But, yeah, I'd also consider putting in some little stakes and actually attaching a line across the yard line (after moving their chairs back over to their yard). As others have said, there's liability for having them in your yard, and making it clear where the fence line is would probably help for when the guys come to fence the yard. I'd think there'd be a halfway decent chance that if you put in some sort of stakes where the orange flags are, then attach a clearly visible line (there is bright pink string, that sort of thing) around a foot high or something, it will make the fact that their yard ends there very clear, and I wouldn't be surprised if that were the end of it without you having to say anything. If they just step over the line... well, then you've got people who are either *very* clueless or who are deliberately using your yard, and the next step might be necessary. When does the fence go up?