aspell is out of the question, it's under GPL
http://www.gnu.org/licenses/gpl-faq.html#GPLInProprietarySystemUltraEdit really uses aspell? In that case, you can demand the source code for their product-- go now.
That also leaves out many other libraries as well. If you know any good proprietary ones, let Khaled know, and maybe he'll find it cost effective to implement.
--but it really all comes back to the fact that many people of many many cultures use mIRC, meaning whatever library chosen would have to support many of those languages to be worthwhile to the userbase as a whole. It would be greedy of you to make a feature request that only the English speaking community can reap the benefits of. And while its likely that there exists such a library that supports many of those languages-- the library would then have to start being packaged with mirc, including all of the language packs, significantly increasing the download and install size-- for what? to know when you mistyped 'thier' in an IRC chatroom full of people typing things like "u r here 2 lol!"? Doesn't sound all that worthwhile to me. How many people even type full proper sentences on IRC to begin with? How can any library keep up with the multitude of IRC acronyms that exist, in every language and flavour.. what will it do about blatant punctuation? Really-- IRC is NOT a medium for proper spelling.
I urge everyone to do a little experiment of their own:
Look at (some of) the active IRC channels you log onto regularly. Count how many times you see a non-English (if you speak another language, replace English with your language) word in a line of text. Include all nickname references, abbreviations, acronyms, (most) URLs, and spelling mistakes both intentional and unintentional in your stats. Sum up the total number of invalid "words" and divide by the number of lines you counted. This won't work for all channels, but it will probably illustrate how much of IRC isn't even <insert-your-language-here>. May make you think twice about what purpose spell checking would really serve
