Although tourism is growing in Peru, there is still very little English at tourist sites – a problem, since English is the language that most foreigners are likely to know. The English one does find is generally bizarre. It's either from 1940's textbooks, rock lyrics or some other odd source, and it tends to be rather abstract.
This shouldn't deter you from going, but if you understand any Spanish (or any other romance language) at all, it's highly recommended that you follow the Spanish-speaking tours and read the museum signs in Spanish. Otherwise, you'll spend most of your time trying to recognize a language you thought you knew well.
For instance, at a museum in Lima, two Germans invited me to join their tour, since they had hired an (ostensibly) English-speaking tour guide. It was painful. The guide kept mentioning the domestication of the "hinnypee", but we couldn't figure out what that was. After careful cross-examination and a LOT of work, it turned out that the "hinnypee" is the cuy,conejo de india, or guinea pig.
Some gems from the Museo Rafael Larco Herrera. These are descriptions of the erotic pottery collection:
"Pain, satisfaction, weep and happiness are all soul-states which were expressed in a realistic and special way by the Mochica sculptor."
"...skeletal persons playing a 'flauta de pan' (Bread flute)."
SUNAT puts out a brochure welcoming guests to Cusco and Machu Picchu. They tell us:
"Macchu Picchu is an Inca fortress, considered unparalleled all over the world, due to its completely inexpugnable situation."
"The travel by train is performed through the National Enterprise of Trains ENAFER PERU, which must give you a ticket."
"In Cusco shops you will find a great variety of artistic samples of our gold and silversmith, paintings, jewlery gems, jewelry, saddlery, weavings and ceramic which enjoy of international prestige."
"In Cusco we have typical dishes such as: corn and cheese, backed sucking pig, cracknels, pickle meat, timpu, etc."
And in fact, there were signs in Cusco advertising "Hog Dogs" and "Chicken Gordon Bleu" (in Spanish, gordón means a big, fat person). Diamond Jim surely would have loved the place.
In any case, the examples are myriad. If you have a sense of humor, and you don't have a year to study Spanish, you get a game show thrown in for free. Besides, non-native speakers come up with some incredibly novel and creative English. Listen hard – you may learn something.