I write most of my email (in gmail) and submit most web site content in English, however, a significant portion is also done in Italian. I leave the spell checker in English, because Italian is, in general, quite easy to spell, so that even as a native speaker, a helping hand is occasionally welcome. However, it isn’t as if I write Italian perfectly either, so the help there would be nice as well. I find it quite annoying to go change the language in the spell checker option each time, especially when, as an example, I’m responding to email and do 2 in English, one in Italian, another in English, and so on.
Ideally, some sort of adaptive technology would be possible, where it figures out what language I’m using, out of a preselected list, after a few words, and starts behaving accordingly.
It seems as if there is some discussion, so hopefully a solution won’t be too far in the future.