We already communicate to the browser that
nl.wikipedia.org is in
dutch, I haven't looked at the firefox 2.0 spell checker but I suspect
they don't do any language detection at all since:
<html
xmlns="http://www.w3.org/1999/xhtml" xml:lang="nl"
lang="nl" dir="ltr">
is *the* standard way to say that a web page (and by extension any
text areas on it that don't have their own lang and xml:lang
attributes) is written in dutch.
With regards to the legal issues Gerard is right, there are none.