I am not sure about what is proposed usage for this application. It works somehow, but I am not sure dictionary is perfect. When I enter "a.p.p.l.i.c.a.t.i.o.n" - it do not offer word "application" itself
Other question is - why one needs to separate letters with dots or commas? You will not mistake one letter for two anyway so there is no need to persuade user doing this - let him enter letters clinging to each others (like in normal word) and then split programmatically.
Also it would be great if you improve usability by providing custom-drawn field for entering word (with bigger letters) and custom graphic keyboard - otherwise system keyboard hides or resizes your window, in the way which you could not predict or manage.
Thanks for good exercise anyway!