OK, new version is up. It will go through the browser languages until it finds one. It doesn't use quality checking, though.
It doesn't seem to do this for me. I tried Basque-German-English and got English text. OTOH, the English wikipedia wasn't highlighted either. So maybe there is a Basque version but the text hasn't been translated yet? Fair enough, but in the real version, we should make sure that we don't count as recognised any language whose text we haven't yet gotten around to writing.
(For the record, lest there be any confusion, I chose Basque at random, since all of the languages that are normally in my browser preferences are already listed on the test page.)
-- Toby