You can talk English in Hesse without a problem, but it's better when you speak slowly, as many people are not confident about their English and do not want to embarrass themselves with a native speaker. In smaller towns and out in the country, it's more likely that you will encounter old people who cannot speak or understand English.
However, as students take English as a second language, you'll find that almost all young people speak English well, albeit possibly accented. Even slightly older people usually do have at least some command of English, and in the cities you should not be surprised to find a 60 year old who speaks English quite well.
You may be surprised at how friendly the people can be, as (like most Germans) the Hessians are very friendly and nice when you are friendly too.
You can get some good tips on local events and places to visit from the locals if your take the time to ask.
Feel free to try out any German you have--either you'll get what you want, or at the least impress/amuse your victim!
German is the main language in most of the state, although Hessian, the local dialect, is spoken natively by many rural and old people and can sound quite different from standard German. However, since almost all Hessian speakers also speak standard German and most people also speak at least rudimentary English you shouldn't have any problems communicating with them.
International visitors will arrive mostly at Frankfurt Airport, the second largest airport in Europe and a major hub for the German carrier Lufthansa. Frankfurt's central station is less than 15 minutes away by subway.
Hahn, somewhat misleadingly officially called "Frankfurt Hahn" even though the city is over 100 km away, is a former military airfield being used by "no frills" low budget airlines. Getting from Hahn to Frankfurt takes about 90 minutes.
There are large regional networks of public transport:
Local specialties include wine from grapes, especially white grapes, and from apples (a kind of cider). This apple wine (Ebbelwei or Ebbelwoi) may be enjoyed straight (pur) or mixed (gespritzt). The latter versions distinguish between "sweet" and "sour", i.e. mixed with either some citrus soda (Süßg'spritzter) or sparkling mineral water (Sauerg'spritzter).
Across the state and the country is a dense network of Youth Hostels (membership required).
Good luck if you're out of the major cities like Frankfurt or Wiesbaden.
Public telephones are rare in many areas, and to complicate matters there was a transition from coins to rechargable/disposable cards a few years before mobile phones made public telephones mostly obsolete. You can buy public telephone cards at the Post or some shops. If you have a mobile phone that takes SIM cards, consider buying a disposable SIM at a mobile phone shop. In the case of an emergency, most people would let you use their mobile phone.
The number for the Police (Polizei) is 110, and for the fire department (Feuerwehr) and ambulance service 112. They can often speak some English.