I'll be the voice of dissent here - I have and will continue to offer partial refunds to guests who are inconvenienced in a way that I deem to have been preventable. I don't know that Netflix being in Spanish would be a deal-breaker for me, but I have found that even a small gesture helps calm guests down.
Instances I've provided small refunds:
-The guests ran out of toilet paper and I had to send them down to my storage locker to retrieve more (the cleaners should have left some extras in the closet, but forgot)
-The building's heat is turned on by property management in mid-October, but we had an unseasonably cold September and I did not yet have a space heater in the unit
-Guests found a HUGE half-smoked joint and ashtray that had been stuffed in a cabinet by another guest, and my cleaners had missed it, etc.
In these cases, I offered varying amounts between $10 and $50 (cleaning fee) as a refund. In your case, I would suggest messaging and saying something like 'I've confirmed with my cleaners that they did not detect any cigarette/other odours when they were present, so I do apologize. It may be possible a neighbour has partaken and you're able to smell it - I'd like to offer you a reimbursement of $20 as a gesture of thanks for your patience and understanding. I apologize for the Netflix mix-up and will make sure future guests set the account back to English when they leave. If you have any further questions or concerns, please don't hesitate to reach out! All the best.'
I'd rather lose $20 and possibly get a repeat guest than have my account littered with bad reviews, but that's just me. I know this is more of a 'business' for some, so their bottom line is all that matters. Good luck!