Blocking the communities? There aren't that many non-English comms so blocking all those comms shouldn't be too bothersome.
You can also opt to block the whole instance, if the whole instance is language-specific. This is a new feature in Lemmy 0.19. This is effectively the same as blocking all comms from that instance; it will not block users from that instance and you'll still see their comments in other comms :)
Is there not some way to just tell Lemmy the languages I speak and have it show me only those?
There is actually this exact feature if you look in the settings. But most posts don't bother specifying a language and the language is not detected automatically AFAIK. So it's not too helpful. But blocking the non-English comms should be sufficient.