Planning to travel to France, Netherlands, Germany, and Italy. Which language is most worth practising?

I've been told I don't *need* to know any other languages other than English but I've never travelled to a non-English speaking country before (I'm Australian and I've only been to New Zealand and Canada), so I want to have at least some words or phrases I know before I go. The trouble is, I'm going to four different countries with different languages.

Am I better off learning key phrases in all four languages? Or should I focus on one? If only one, which one? And if I'm better off learning a few phrases in each, what phrases should I try to learn?