Probably. However, establishing that the "lang" attribute is the "first-choice" language to check (which wouldn't prevent the UA from providing other choices, or just ignoring such behaviour due to a user preference, or using other dictionaries too -- and that might be suggested in a note on usability, I guess), I mean, would allow a webapp to emulate those functionalities to some extent, just setting a different value for the lang attribute of a contenteditable box and some of its subregions through a script at the user whim (that is, let's do it through script until UAs provided a better solution, which could be hinted by scripting hacks based on the "lang" and "spellcheck" attributes working together at the same grane).
I don't think that applications need ability to precisely control spell-checking language. Browser knows best which dictionaries are available, and can auto-detect language based on user's preferences, page's language and text itself. You can expect that browsers will have much more sophisticated and reliable language detection than web apps (that's an area where browsers can freely compete).
Many of your suggestions are just implementation details, which HTML shouldn't specify precisely (it could force browsers to use method that is suboptimal). HTML just needs to offer reasonable way to implement good heuristics, and I think existing lang, input types and spellchecking attribute are sufficient.
-- regards, Kornel
