I went through itypedef.cpp and corrected every typo that I saw, plus one or two grammar fixes. I also removed all the double spaces after punctuation marks, because it’s bad form. 
Attached are the diffs. I had to split the changes into two files, as the original file exceeded the forum memory limit on attachments.
spellcheck1 (73.5 KB)
spellcheck2 (78 KB)