Most people in the world do not speak one of the uber-languages (english, chinese, spanish etc). These people are particularly poorly served by a drive to document in their native language rather than a common language. If someone only speaks Xhosa and never gets to exercise a documentation-common-language, then they'll only be able to read and contribute to documentation in Xhosa, which is a much smaller community than 'everyone'. And what if the leading engineer in your field didn't document in that language? You'd be cut of from what they had to say.
A common documentation language functions as a technical standard, like IPv4. Sure, you can use your own standard, perhaps IPX, but you'll only be able to communicate with a much smaller group of people.
It's hard to get documentation written in the first place, let alone good documentation. Making it 'fair' by saying that documentation has to be effectively splintered into every native language is a sisyphean task.
As always, write to your audience. If it's a general tech audience, that means English. If it's Chinese engineers who don't speak English well, that means Chinese. But those Chinese engineers are going to have an easier time interpreting foreign documentation if it's all in one language rather than 50.