This one is only tangentially related to fraud prevention. But I've never really understood this. Is the Accept-Language header really so inaccurate?
I'd say so yeah. Many many old people here (in a country where English isn't the primary language) use 'misconfigured' browsers.
It's getting better nowadays I suppose with new installers defaulting to our native language but (even though I tend to prefer the English version myself) I think it's a good default. I do prefer an easy way to change it though (or a quick confirmation on the first visit.)