How easy is it for an English only speaker to navigate Iceland [duplicate]

Depends on what you mean by "navigate." To get from place to place, a map or GPS should be sufficient.  For anything else, Spanish will be of little value (although I did manage to communicate with an Italian restaurant owner).  However, English and Danish are required subjects in the Icelandic schools.

Road signs will not be in English, but the meaning of "80" is pretty obvious, and many of the symbols, shapes, and colors you are used to are international. Destinations will be their local name on signs at exits, but if you are looking for a particular place you already know its name.

If you already know some of the places you'll be, use Google Streetview to simulate going down the street and looking at the signs. Or http://walkscore.com to see what sorts of businesses are nearby

Lots of touristy marketing brochures will be available in several languages.

It's not hard to identify a restaurant, even if you can't read the menu. Or hotel.

In Iceland, everyone speaks English (except maybe an old man in a village far far away). For the rest see the WGroleau's answer.