In the docs for CGI::FormBuilder::Messages is

my $form = CGI::FormBuilder->new(
  messages => {
    js_invalid_start  => '%s error(s) were found in your form:\n',
    js_invalid_end    => 'Fix these fields and try again!',
    js_invalid_select => '- Choose an option from the "%s" list\n',
  }
);
[download]

It appears to be more than just choosing a locale. There is a list of available messages and their defaults.

hth