I have made a small library to ease changing and translating the error messages. You can even change the texts by error type which is currently not available using title
in Chrome or x-moz-errormessage
in Firefox. Go check it out on GitHub, and give feedback.
It's used like:
<input type="email" required data-errormessage-value-missing="Please input something">
There's a demo available at jsFiddle.