Although the official language in Greece and Athens is Greek, English is also widely spoken, so you shouldn't experience any problems when visiting the city. English is very widely spoken in Greece, especially in the most touristy parts of the city.
Can you drink tap water in Corfu?
You are advised to drink bottled water in most areas of the island. It is perfectly safe to drink tap water, but it is rather “hard”, i.e. it has a high mineral content, therefore you may want to drink bottled water instead. A bottle of water is available in every room upon arrival.
Which is harder Greek or Spanish?
Greek can be a slightly challenging language to learn. For English speakers, it's certainly going to be more difficult than languages like French, German and Spanish, but much easier than languages like Mandarin or Korean.