It is important to understand first that what normally is called “international driver’s license” is not really such a document, but it is actually a translation most of the time into English and maybe into some other languages from the original driver’s license. This document known as IDL (international drivers license) does not substitute the national license and it should not be used as a means of identification, even in some countries such as United States, an attempt to use and IDL as primary means of identification is against the law.
Having said that the question is if one really needs an IDL when traveling abroad.
Rules in this issue can change from country to country, but it is generally accepted that a drivers license in English is enough to rent a car in most countries. Normally Americans or Canadians and citizens from other English speaking countries do not need an IDL. It is more common to request an IDL from non English speaking countries.
At this point the conclusion is that the best and most prudent thing is to carry always an IDL when traveling. This is a document that is easy to obtain, at least in the USA it is affordable and many online websites offer to process one of these. In other countries it can be somehow confusing to know who is in charge of emitting an IDL, normally they are associations of drivers or clubs of automobile fans.
If one wants to avoid obtaining an IDL, but is still thinking to rent a car abroad the best thing may be to call the car rental company ahead of time at the destination and inquire about the particulars of that country.
Please do not forget to always carry with you three documents when driving abroad: Your passport, your original driver’s license and your IDL.