When this text was written, visiting http://en.wikipedia.beta.wmflabs.org/wiki/Europe in incognito mode (logged out) returns wrong accept languages:
mw.config.get( 'wgULSAcceptLanguageList' ) ["ru", "uk", "be", "en", "*"]
For the record my accept language header is:
My theory is that this is because anonymous page views are cached at Wikimedia.
We should stop outputting wgULSAcceptLanguageList for anonymous users if language selection is not enabled for anonymous users.
There is no way we can get this data always for anonymous users for compact language links in the sidebar. We can get that list by doing an additional request that is not cached: for that we should probably create a new API module in ULS or core. We could also store this data on localstorage or cookies (to share between sites) for some time to use it for the future requests.