How to change validation error messages

There are default error messages such as "Please fill out this field", if the field is set as required but the user leaves it blank. Is there a way to customise these messages?

That message is generated from the browser when the word required is on an input (that supports it), it's not actually part of Bootstrap. I believe that it is an HTML5 thing though.

To do what you want, you would have to create for yourself a FORM validation script in which you can then display whatever message you want for errors etc..

I'm sure that you can search for a JS plugin to use, there a quite a few to choose from. Since JQuery is already included and if your familiar with it, I'd suggest looking for a plugin based on using JQuery.