I have rather less experience of Spain but have found a good map and language apps help enormously. At least they use same alphabet, which of course Greeks do not.
Athens on the other hand is very easy to get around once you get used to it, and an easier, relaxed culture than most parts of Spain that I’ve been to.
I would maybe suggest at least becoming familiar with the Greek alphabet, at least the letters that have funny shapes, and common substitutions such as using B where we would use V (as in Biktoria instead of Victoria). You want to be able to decipher a Subway (Metro) sign yourself, especially if you arrive from the airport at Syntagma in the rush hour. Athenians are mostly polite, friendly and honest; but there are a considerable number of immigrants inly too happy to relieve you of your possessions. I use an iPhone openly in some areas but not in others, such as Omonia (a busy interchange) or on a crowded Metro (station or train). Most of the basics are on the UKP Wiki.