If this is too complicated, it is the fault of the user agent. That's a different issue than whether a website should respect a requested language if it is avai

If this is too complicated, it is the fault of the user agent. That's a different issue than whether a website should respect a requested language if it is available. You're arguing that the user agents are flawed, so we should have flawed content delivery to compensate. This is how flaws get folded into systems and magnified over time. Just fix whatever flaws exist in the user agents, language selection based on http headers is a pretty good solution for the half of the problem it covers.