For that matter, there are human languages besides English that can be written using only ASCII (sometimes by transliterating) and that's not what happens in them.

Just like people know they have to write in English on HN to communicate, they also write their code in English when they plan to open-source it and share with the rest of the world. As for closed-source projects ... if your company doesn't conduct its business in English, why force the code to be in English? The only people whom that'd benefit are never going to see it.