What is actually a tech failure, is assuming I understand Spanish if I happen to be in Spain. That sort of ham-handed, short-sighted approach drives me nuts.
You only need "fiddling with language settings" if you want something other than how your operating system is configured.
Google actually did this at one point. I was in the Netherlands when I signed up for Google+. I don't speak Dutch and hadn't changed any of my language settings - the only way they knew was via my IP address. The UI was mostly in English, but all my circles (or whatever they were called) were named in Dutch.
I get the weirdest language bugs with Google:
- The info box on the right hand side of the search results intermittently shows up in German
- A suggested event (a football match) in the UK (where I am) had both team's names transliterated into Cyrillic
- An airport on Maps consistently shows up in either French or Finnish (or both!)
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.
They shouldn't have to fiddle with language settings. That's a tech failure.