The main language is Croatian, a Slavic language very similar to Serbian and Bosnian.
Many Croats can speak English to some extent, but German and Italian are also very popular (mainly because of the large annual influx of German and Italian tourists). Older people rarely speak English, although they can converse in German or Italian. If you know Polish or Czech, these languages have some similarities with Croatian. Some people also speak French or Russian. Many older people can speak Russian, but among the younger generation this language has been largely displaced by English.