Translate Calendar

Don't modify any Gravity Forms files.

I would put the language file in your child theme folder, in a js folder beneath that. Something like this:

/var/www/public_html/example.com/wp-content/themes/child-theme/js/jquery.ui.datepicker-es.js

Then, you need to include that in the footer of the page where the form is loaded.

Use the WordPress wp_enqueue_script function to make sure the script is loaded in the footer and after jQuery and the jQuery UI datepicker script is loaded.

That will load the translation file in the footer of your page. View the source of the page and find the script link, then try accessing the file by URL in your browser to make sure you have the path correct.

The additional JavaScript to make the language choice global will be added to the header (or footer, I think) section of the page. You can save this script to a file, and enqueue it in the same way:

jQuery(document).ready(function($) {
    $.datepicker.setDefaults($.datepicker.regional['es']);
});

Let us know after you have tried that and link to the page with your datepicker on it if you need any additional assistance.

Thank you for the update.