"Fregley's Language" is a language made by Fregley.
Very little is known about the words in this language, but they seem to consist of words already used in the English language, just used for different purposes. For example, whenever Fregley has to go to the bathroom, he says "Juice! Juice!" This example is shown in both the online book and the first published Diary of a Wimpy Kid kid book.
Greg says that the kids have pretty much figured out "Fregley's Language" but that the teachers still haven't caught on. It then shows a teacher bringing Fregley HI-C. In the published version, a different teacher (Mr. Underwood) is shown.