Funky code in Views UI to make Add display list doesn't work in non-English languages

And by the way, it doesn't work in Catalan either. In this case, the word "Add" apparently comes first in the strings, unlike Hungarian where it comes last, but it is still not removed from "Afegeix Bloc" etc. See screenshot:

Here's the code that is doing this. It's in core/modules/views_ui/js/vies-admin.js in class/objectDrupal.behaviors.viewsUiRenderAddViewButton:

      var $addDisplayDropdown = $('<li class="add"><a href="#"><span class="icon add"></span>' + Drupal.t('Add') + '</a><ul class="action-list" style="display:none;"></ul></li>');
      var $displayButtons = $menu.nextAll('input.add-display').detach();
      $displayButtons.appendTo($addDisplayDropdown.find('.action-list')).wrap('<li>')
        .parent().eq(0).addClass('first').end().eq(-1).addClass('last');

      // Remove the 'Add ' prefix from the button labels since they're being
      // placed in an 'Add' dropdown. @todo This assumes English, but so does
      // $addDisplayDropdown above. Add support for translation.
      $displayButtons.each(function () {
        var label = $(this).val();
        if (label.substr(0, 4) === 'Add ') {
          $(this).val(label.substr(4));
        }

So. The comment here is incorrect, because the variable $addDisplayDropdown a few lines up is actually translating the word "Add".

And what really needs to happen here is ... Hm. The label here is actually coming from a call in PHP to t('Add @type') with the @type substituted in from a call to something like t('Block') or t('Page') or t('Attachment') [the defined types of displays you can add]. So the JavaScript needs to call Drupal.t('Add @type') and then figure out where the "@type" appears in the string for the current language. Then it can extract that out.

For instance, if the translation of "Add @type" is something like "Foo @type barz", then it would need to take a substring that chops off the first 4 characters and the last 5, to find the "@type" portion of the label. If @type is at the start, then don't chop off anything at the start; if it's at the end, then don't chop off anything at the end. Pretty easy to calculate and achieve, using the JavaScript substr() method on strings. str.length(4) would take everything starting at character 4 to the end of the string, and str.length(-5) would take everything but the last 5 characters.

Let's see... how about this patch? I'm testing it in English, Hungarian, and Catalan by making User Guide screenshots, and will posts them when the tests finish. Or post a better patch if it doesn't work right. ;)

That patch wasn't quite right. Here's a new one that works for English, Hungarian (where "Add" is a suffix), and Catalan (where "Add" is a prefix, but not 3 characters long). And some screenshots.
English:

Hungarian:

Catalan:

Adding some tags.

Also note that the CSS for this drop-down needs some adjustment probably, but that was present in the "Before" screenshots in the issue summary, and fixing it is probably out of scope for this issue. (The left side of the buttons in the drop-down list is cut off for some reason by the CSS.)

Yes, it definitely makes the JS code much less funky! A good thing.

You'll need to have the modules language and locale enabled.
Then in your test you can go to admin/config/regional/language/add and submit the form there with drupalPostForm, using something like

$edit = [
  'predefined_langcode' => 'es',
]

and hitting the button 'Add language'.