x-default pointing at the home market
Ours pointed at French for months. That tag exists for visitors whose language matches none of the declared alternates, so a reader in Japan or Brazil was being handed French. English was the sensible fallback all along: it is the version most likely to be understood outside the declared set, and it carried most of our measured traffic. The fix is one line, and noticing the problem at all takes a deliberate look, because nothing is broken and nothing reports an error.