Comment 8 for bug 303269

(In reply to comment #7)
> nsEditorSpellCheck is no service and you get one instance per input/textarea
> element for which spell checking is possible. InitSpellChecker is thus called
> once per element and would thus be the appropriate place for figuring out the
> page's default language.
>
> You should be able to start searching from aEditor->GetRootElement. Make sure
> to consider HTML's lang attribute and xml:lang (whichever is appropriate) and
> as a bonus it would be nice to also select not completely specified languages
> (e.g. select es-ES for lang="es").

Definitely take a look at how the spellcheck attribute support is implemented as well, since this is another case where the editor needs to do the right thing based on page content. (Although that's much more complex than this, in that it needs to support all kinds of bizarre dynamic behavior and overriding.)