Listen to Music off Watch while on airplane mode

How to sync your music to your Apple Watch

https://support.apple.com/en-us/HT204691

Since you do not have a data plan when in another country, you have to have music stored on your Apple Watch.

This can be music you own and have downloaded to your iPhone, or you copied from CDs (remember those?).

I know Pandora paid subscribers can down load off-line play lists to their Apple Watch. I do not know about Apple Music, and I'm fairly sure Spotify does not offer off-line listening.

You can also store podcasts on your Apple Watch, and while Apple does provide an Apple Watch for that, I recommend a 3rd party podcast app for your Apple Watch, as they manage the podcasts much better and make sync'ing new podcasts easier.

I am fairly sure you can get an Audible app and listen to audio books on your Apple Watch (this I have not done; the above 2 I have).

Assuming you have music or podcasts stored on your Apple Watch, you would

Turn on Airplane mode via Apple Watch -> Settings -> Airplane Mode

Turn Bluetooth back on via Apple Watch -> Settings -> Bluetooth

Use the Music app or podcast app to play content from our Apple Watch to your Bluetooth headphones.

NOTE: The suggestion to use your iPhone has some benefits. You can use corded headphones, so you do not need to worry about using a radio to drive you headphones. The iPhone battery will last longer than your Apple Watch battery (if it is a long distance flight). You can store a lot more music and podcasts on your iPhone.

Just use your iPhone...Some airlines let you keep your iPhone turned on if you switch to airplane mode. You can't make calls or use Bluetooth, but you can listen to music, play games, watch videos, or use other apps that don't require network or phone connections.