This follows up on
this thread from January 2014 and
this thread from February 2015. (Yes, this means I’m two months late with this thread.)
As the other folk have noted, Sigil’s spellcheck has trouble with words with Unicode curly apostrophes (at least on Windows). I’ve found a partial work-around: when I want to add a word to a personal dictionary, I first replace the curly apostrophe with a straight one—this lets Sigil add the word to the dictionary, and thereafter it will not flag the word even with a Unicode apostrophe. To those familiar with the spellcheck-interface code, this could probably suffice to point to a patch: when adding a word to the dictionary, replace curly apostrophes with the straight ones.