Comment 39 for bug 303269

(In reply to comment #7)
> (In reply to comment #4)
> > But it is my understanding that this nsEditorSpellCheck::InitSpellChecker() is
> > called only once per browser instance?
>
> 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").
>
> (In reply to comment #2)
> > but it'd be nice to have for those bilingual people out there.
>
> Of which there are an awful lot on the English dominated Internet of a polyglot
> world. :)

Unfortunately, even if it's not too hard to check the lang that has to be used during the call to nsEditorSpellCheck::InitSpellChecker() (for each editor's instance), the problem is quite hard because there is only one instance of nsISpellChecker (implemented by extensions/spellcheck/) which is used to set the dictionary. In other words, changing the dictionary for one editor will propagate the change for all the editors. That's exactly the behavior you have when you change the language with right click -> Languages -> <language>. And that's exactly what we don't want for @lang. Actually, we may don't want that when changing the language manually too.

On top of my head, I would say the best way to fix this bug would be to set the desired dictionary when the editor got the focus so it could be possible to set the dictionary to the language corresponding to @lang and still be able to revert it when using a field with another @lang or with the default one.