Rob Allen has posted a quick hint about overriding validation messages in a Zend Framework v2 based application. This override is related to the output of a standard form and works globally instead of just on a single form.
One thing that I always do when creating a Zend Framework 2 form is override the validation messages for a number of validators - EmailAddress in particular. I recently decided that I should probably sort this one out once and be done with it. Turns out that it's quite easy assuming that you use the FormElementManger to instantiate your forms.
The post includes all the code you'll need to do the override: a custom validator example, the changes you'll need to make to the configuration and an example of a form that uses the custom handling. He explains each of the parts too, showing how they fit together in your module.