When do you need an international drivers license?

always

No, it depends on where you are licensed, and where you are going.

First off, let's clarify the "International driver's license" (more accurately, international driving permit [IDP]). It does not

allow you to drive a car anywhere in the world, all by itself. It is only a translation of your regular drivers license into multiple languages, and it's no good without your legally-issued drivers license. It is not valid in your home country.

The main reason for it is so that police in Japan or Saudi Arabia who can't read English will be able to see what your drivers license says.

It is valid in all countries that signed the 1949 Convention on Road Traffic. Other countries that did not sign the agreement (such as Mexico) still honor the IDP. Some (like Brazil and China) don't recognize it at all.

If you come from an English-speaking country, your regular driving license will be OK in other English-speaking countries such as Canada, the USA, Great Britain, Australia or New Zealand. Most of Europe will also accept a North American driving license.