This works really good - I was amazed by the idea to enable corrections with drop down boxes, nice.
But PLEASE - make the default "textarea 0 richtext 0". 99% of the normal text inputs make no sense to be spellchecked. (E.g. the value of the parameter setting itself 😉).
I wonder how this works with internationalisation - shouldn't it choose the language according to the users locale? Or - if no dictionary is available for the current locale - don't offer to spellcheck? I just tried spellchecking some german text with this feature and the suggested corrections made no sense at all of course.
Also it needs to be added to the installation instructions either, or it needs to check wether the spellchecker binaries are available and don't offer to spellcheck if not (which I'd prefer to be honest, propably it can be cached easily).
And a minor issue I noticed: when entering text with "quotes" in it, it messes up the error message somehow - it is not red anymore and not separated by a line break from the corrected text. Shouldn't the text outside the select widgets be html quoted?
Finally a little suggestion: I think it would be nice in case of spelling errors found, to offer the option to re-edit the whole text in a textarea again, in case the user wants to rearrange the text, change some words etc. This is not always achievable with the back button, e.g. when the form has been reloaded a few times already because of other input errors, so a little additional checkbox for that wouldn't hurt.