I've been to Germany many times as an exchange student and to visit friends, my experience of Hamburg and Cologne international airports was that they spoke English just fine, I doubt Frankfurt Intl is much different.
Mainland Europe has a substantially better education in foreign languages than England does, as a rule most people you encounter there will speak at least two languages.
I believe it's actually a major factor in being hired at international airports that you speak as many common or uncommon languages as possible, English being a pretty high priority.
If, by some terrible run of bad luck you encounter the one member of passport control who doesn't speak English, their first reaction will be to fetch someone who does.
In short, Don't worry about it. You'll be fine.
What you should definitely be thinking about is your over-land travel arrangements to get from Frankfurt to Würzburg, I recommend compiling some common and emergency phrases on paper.
You'll probably still be fine, but the further east you go in europe the less likely the people you meet are to speak fluent english. Particularly bus-drivers and shop-keepers.