I've recently decided to give Oblivion another whirl, and saw an expansion pack called "Shivering Isles" on Amazon so have picked that up.

Unfortunately when it arrived today it looks like it's a French language version of Shivering Isles which has been sent by mistake (I was expecting English from the website). I've not yet opened the Shivering Isles box because of this. My Oblivion game, which is the language I'd like to play in, is in English.

My question is whether it's possible to download an English patch or something else to change the Shivering Isles text (and any spoken dialogue, if there is any?) to English, or do I need to send it back and get a refund? I did a search on an Oblivion wiki I found, but no answer.

Nope. Each copy is language specific. You need to send that one back and get the english version. I don't even know if two different language versions will even work together (although I suspect they would).

Thanks for letting me know--that's a shame!