The Basics
My face when X call Y Z is a phrase that has begun appearing on the /v/ board of 4chan. The main purpose of the phase is to incite rage or laughs in whomever reads it. It involves poking fun at another nation or group of people by implying that they using a made up or nonsensical word to replace what the actual word is. Most phrases are accompanied by the image of a shocked person or someone laughing smugly.
An example of this would be "my face when Americans call Wacky womper tallywags 'TelePhones'"

The X
The X part of the phrase refers to a nation or group of people.

The Y
The Y part of the phrase is what the true name of the word that X is referring to. It is commonly a made up and silly word. This is implying that Y is the actual name of X and anyone saying otherwise is wrong.

The Z
The Z part of the phrase is commonly a real object or action. The joke of the entire phrase is that what X calls Z is actually not what it is supposed to be called.