theme('system_date_format_localize_form')

+++ b/core/modules/system/system.module
@@ -208,10 +208,44 @@ function system_theme() {
+      'template' => 'system-date-format-localize-form',

We don't need this, as we already look for template files by theme hook name (after replacing underscores with dashes).

+++ b/core/modules/system/system.module
@@ -208,10 +208,44 @@ function system_theme() {
+  * Default theme implementation to display a locale date format form.

Make this read "Preprocess variables for locale date format form templates."

+++ b/core/modules/system/system.module
@@ -208,10 +208,44 @@ function system_theme() {
+  *
+  * $variables
+  *   An associative array containing:
+  *   - form: A render element representing the form.
+  *
+  * @see system-date-format-localize-form.html.twig
+  *
+  * @ingroup themeable

Remove all these lines.

working on getting '/admin/config' fixed before I can review.

Needs some cleanup in the template docblock. I will finish this up.

Also have a go at the theme() function so it doesn't throw errors due to the new stuff it's getting from the preprocessor. (Hint you won't see errors in bartik because of #1855520: System status messages missing from Bartik)

What's a path for viewing this theme output?

It seems it should be something like admin/config/regional/date-time/formats/long/edit, but it's not (apparently).

Thanks, artofeclipse. I'll be able to check it out later today.

sorting doesn't work.

@steveoliver the system_date_format_localize_form function only affects the edit locale form format. For example: admin/config/regional/date-time/locale/en/edit (see the attach)

This table is not sortable.

OK, Cool. Let's roll this into #1898454: system.module - Convert PHPTemplate templates to Twig. Could you please post this patch to that issue?