There are now many Americans a Canadians, living and renting properties on Isla Mujeres and speaking Spanish is not essential on Isla; almost everyone has some English, certainly enough to direct you to where you want to go or make your purchase. If you do speak some Spanish, Island people are very patient and they will help you whilst you struggle with your English Spanish phrase book. In Spain waiters for example have no patience with customers broken Spanish but in restaurants in Isla Mujeres you see Americans trying to order in Spanish and the waiters, even if they speak perfect English, wouldn't dream of speaking Spanish.
The favourite means of transport on the Island is a golf cart, they are not cheap to rent but they are a great way of seeing the Island. Most of the luxury Isla Mujeres real estate is at the southern end of the island in a golf cart it takes about 20 - 25 minutes, so if you are buying or renting at Punta Sur, going down to the centre to dine it's very convenient, going to the centre, twice a day in a golf cart can become a bit of a drag. But you don't need to do that anyway, you can relax around the pool go down to Oscar's for lunch which is not far away where they do some great Pizzas, and go into the centre to dine. The centre can at times get a bit crowded but it's a great place to spend the evening, so many bars and restaurants with open air dining.
Taxis are not expensive but you do need to clarify the price before you get in, it isn't the best of ideas to get to the other end and get a surprise. If the fare you are given seems a bit high you can usually negotiate, obviously if it is pouring with rain and there are no taxis about your negotiating position is a bit weak. Don't be surprised if they pick up other passengers along the way, it is standard practice on Isla Mujeres. Locals do pay a different price from foreigners, it is understandable to some extent; they see you coming out of a $1,000,000 Luxury Isla Mujeres home, and take the view that a few more Dollars won't hurt you. In any event they will be cheap by US standards.