After Twitter killed the public timeline, users' experience became pretty much limited to the language of the filter bubble they live in.

I understand that some of us make no use of other languages in the federated TL, but complaining that people on Mastodon write in their own language is useless.

If you want to have an English-only feed, don't ask for filters in the federated TL – rather, follow people you find interesting and build the TL you like. It's also beneficial to your instance.