Before going to Europe, please get the app google maps or CityMaps2Go (if you don’t already have it). It is easy to download whatever area of Europe that you will be traveling to.
If you don’t know how to speak any European language, I would recommend downloading the app Duolingo (it’s free) because it can teach you some of the language before your trip.
I always have storage problems on my phone because I take one too many pictures. If you get a chance (and you have low storage), I would recommend deleting pictures (or whatever else is taking up space on your phone) so that you have enough room to take photos while travelling.
Download European transportation apps and use Rome 2 Rio to get the best travel routes.