I wish I'd been better at learning French at school
It would be great to be able to read French, Mediaeval Irish, Irish, German, Latin, Classical Greek etc. Sometimes I struggle with American, but the newer Kobo FWs have nicer USA & British English definitions. Sometimes the odd Australian & NZ.
I find Americanisms in Fantasy novels very distracting, not spelling like color, but USA only types of foods that are unfamiliar or adjectives never used in European folktales. However I'd object to USA books being translated to British English like USA publishers do to British or Irish English titles. I can figure it out.
I think Maurice Leblanc's Lupin is more fun to read than Agatha Christie's Poirot.