It’ll probably be best if you translate inside the controller for this one, unless you want to pass element and minlength to the template.
Firstly you’ll need inject $translate into your controller. Then to generate your message:
this.errorMessage = $translate('MINLENGTH', { element: element, value: minlength });
This method is also outlined here.
To do this in the template (outlined here):
{{ MINLENGTH | translate : { element: element, value: minlength } }}