You can get by in Nicaragua even if you only speak English because of the closeness of our culture to America dating back 25 years. Many Nicaraguans have moved back from the US in the past decade. These people have families; they have children and they kept both Spanish and English as the languages spoken in their homes. Now, many businesses require people to speak both languages. Even the cab drivers here in Managua can speak some English. A quarter of a million people have moved back from the US, which is why the chances are that even the cab driver speaks English.
The people in the Caribbean coast in Nicaragua are descendants of British people, so their native language is English. 1/3 of the country speaks English as their mother tongue. At least half of the people you will bump into on the streets of Managua know how to speak English.