Slovak is a western Slavic language, closely related to Czech. The two are mutually comprehensible. For centuries Hungarian was the official language in much of the territory of modern-day Slovakia, though command of Hungarian these days is limited mainly to ethnic Hungarians in the south of the country.
English speakers are likely to find the language something of a challenge. English is not widely known outside the larger cities, though nearly anyone can muster a few words in a pinch. This shouldn't deter you. Most museums and restaurants and anywhere else you're likely to go will have English information on hand. Almost every tourist information center and hotel will have a least one English speaker. If you know some German, that may help as well.
