Surely it's better to write the speech of the character as difficult to understand rather than just describing them as having a confusing way of speaking?
There's a fine line to hold here. Remember the reader has to endure this "accent" for a whole book. Being confused for better portions of a book flows right into being down right annoying. EG Jar Jar Binks. And will probably lead to the book having more flight time than a 747 because people will be throwing it against the wall in droves.