There's a long tail of mobile browsers out there. I also don't know how many people have OS settings that differ from their real language preference due to poor OS support for their language (e.g. what this article talks about - how many users run en-US to get features that Win10/Android/etc doesn't do well in their language)

I mean, there are reasonable arguments either way here IMO and without actual data, e.g. how often users change their language from one or the other, it's hard to know what is better in the long tail.

And obviously users should be able to change their preference if we get it wrong.