Hi @Koko22 and welcome to the Community! :)
If you download your playlists to your phone it won't use your data plan as long as you keep Spotify offline. Spotify will need to connect online at least once every 30 days though. I've copied over the reasons below from their offline support FAQ.
I hope this helps and that you have a safe/happy trip! :D
Why do I need to go online every 30 days?
Premium subscribers must go online with Spotify at least once every 30 days. This is so Spotify can check that your subscription is still active and count track plays to compensate artists.
If you don’t go online with Spotify at least once every 30 days, any tracks you have downloaded for offline listening are automatically removed from your device.