It's easy to start talking about something in a different language.

It's possible to translate most of the relevant literature into that language.

Changing the keywords in programming languages, HTTP headers, JSON field names, identifiers in library code, error messages embedded deep in boot ROMs and so on is much more difficult.

You seem to think that one needs to learn the human language to learn those keywords. That's demonstrably false, as evidenced by hordes of programmers who can barely read English sentences, much less write anything coherent. They already cargo-cult the programming language words without understanding the human language, and the actual spoken/written languages bear no influence on that. If Spanish becomes the dominant language of the web, some of them will learn to read and write Spanish while continuing to cargo-cult English programming keywords into those conversations.