Because English is the lingua franca of the software industry, and developers are usually expected to know English, no matter where they're from.

The point of my comment was, that shouldn't necessarily be the case forever, and it might not be reasonable to expect it to be.

Sure, nothing ever stays the same, but I think that for the foreseeable future, we can reasonably expect English to stay the universal language of software development. It's the default foreign language people learn in their home countries for a lot of reasons, so a lot of people who want to get into the field already have at least a basic understanding of English. This aids them in learning and communication with other developers, and it's just too convenient to be displaced any time soon.

Because it is a lot of work. Nginx always had good Russian docs, for reasons you can guess.