Obviously users need to add an email address to register an account.
I'm using the Bartik theme. In Firefox and Chrome, if I don't enter a value for the email address and press submit, the form is not sent; instead, I get a JS popup, "Please fill out this field."
When I search the translation UI, I can't find this string ("Please fill out this field.")
I also can't find the string when I grep for it over the entire D8 git repository.
How do I translate this string?
This message is coming from the browser and not from Drupal, so, there's not much we can do about this, except for not using HTML5 required attributes.
I have my system in French and the message comes in French, and I don't have the French language installed in my site.
Read more about this in the Drupal core issue about "HTML5 validation is not fully accessible".
EDIT: It seems that we have a working patch that solve the problem.
You can set it in Javascript with
$('input[type="text"]').setCustomValidity("This is a custom message");
TypeError: jQuery(...).setCustomValidity is not a function; It should be called like this: jQuery('.ed_form input[type="text"]').each(function(){jQuery(this)[0].setCustomValidity("This is a custom message."); }); as setCustomValidity is not a jQuery method but a plain vanilla JS one, so it should be called against a JS object not against a a jQuery one.