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The Firefox browser now has a built-in page translator that works even without the Internet
(gadgettendency.com)
This is a most excellent place for technology news and articles.
So set whatever you want to see. Am not sure what's your point. That setting tells site which of the languages you'd like to see and if they support it, it should be used. Didn't see any sites use these apart from CMS I wrote.
Maybe I misunderstood you but my point was if it interpreted the language preferences I set in the normal config as "knowing" the languages I added and didn't offer translations, that wouldn't necessarily be what I want.