Replying with a different account with a different language setting to see if it helps:

This is most likely because of language settings, and I have to look into it I guess. Lemmy language is such a strange thing – this Lemmy instance has had “undefined” as the only option for a long time. I changed it last night to include English as well…

Try checking your account settings and make sure English is selected. This is the default for new registrations (I think), but older registrations like us old timers have to manually enable it.