I do not work at Google, but I speculate that this is a case of software architecture leading the product: For fast results and scalability Google will just DNS

I do not work at Google, but I speculate that this is a case of software architecture leading the product: For fast results and scalability Google will just DNS-redirect you to the closest servers, and those servers are configured to a specific language. Sure, they could work on it, but for Google engineers cache is king, and for optimal performance they don't want to bother on personalizing that first access.