Some widely spoken language I imagine, Chinese, Spanish, English I don’t care. Since .com is intended for commercial use, the language of the companies biggest market makes sense here as well.
You’re also forgetting that the likes of google.ru, google.nl and google.every_other_country_code exist.
Also there are plently of websites the have language selection in the site that overrides that header, look at Wikipedia.
There are plently of sites in non english languages that cater to non English speakers only, not every site has or needs 10 different translations.
At this point we also have translation engines in the browser so for pages in languages you don’t know, that you absolutely need to access, you can use it to understand the page to a decent level and/or be able to navigate to a version in your language if available.