If you change the language to ɥsᴉꞁᵷuƎ (ɯopᵷuᴉʞ pǝʇᴉu∩) all translated text is turned upsidedown. Sadly this does not affect untranslated text. I am very sad.
[media][media]Custom item names, chat messages, usernames, etc. are affected by this issue.
I hope this issue will be resolved, because it makes reading the chat very hard when playing with my monitor upsidedown 😞
The "Upside-down English" language option does only change the interface language. This is a feature request, not a bug.