You shouldn't have to change this setting multiple times when you're in a country that speaks (well, writes, really) a different language than your computer is configured for. You would like the web to respect your configuration.

On the otherhand, the general public shouldn't have to change this setting multiple times when they're in a country that writes a different language than their computers are configured for. They would like the web to respect their location. Even though they imported a computer configured for English and don't know how to (or don't want to) configure the language options for the whole computer or the browser.

If language settings were easily and often set, they'd be respected more; but if there's a good chance people are more likely to want a local language than what the computer thinks they want, then ignoring the preferences is the right thing for most users. Even if it's infuriating for those it missed. When I had the fortune of working on multi-language sites, I would try to have each site have their own urls, but some places insisted on one url, multiple languages. Trying to make a reasonable initial language decision and then a language picker with a cookie was the best I could do.