The question is, was this a conscious human design decision or did the algorithm learn to do that by itself?
English is used as a lot as a fallback language for inter-cultural exchanges. In that sense it's kind of dominating, but that's it. Intra-cultural communications happens in local languages, and even if that preferred language happened to be one of en-* locales, that only means everyone is functionally bilingual, and it doesn't mean cultural informational borders don't exist. Data still only goes through bridging connections.
So essentially both I guess?
Mastodon only had the raw feed and that drove European network operators insane, so much so that they effectively GFW'd itself.
I would lean for the latter, the simple explanation may be that people just prefer local content.