diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php
index 7d36998..1ba5a3c 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php
@@ -180,6 +180,11 @@ public function viewElements(FieldItemListInterface $items, $langcode) {
       }
 
       if ($format_string) {
+        // Pass in the current output as an override for @count in plural
+        // formatting. The numeric value is used to decide which plural form to
+        // use, but we want to substitute the current number-formatted
+        // string with the thousands separator and other settings applied in
+        // the output.
         $output = PluralTranslatableMarkup::createFromTranslatedString($item->value, $format_string, ['@count' => $output], ['langcode' => $this->getSetting('_settings_langcode')]);
       }
 
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/NumberWidget.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/NumberWidget.php
index 5bcfb7c..da19d5c 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/NumberWidget.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/NumberWidget.php
@@ -116,9 +116,9 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
     // Add prefix and suffix.
     if ($field_settings['format_plural_string'] && $this->getSetting('format_plural')) {
       // The field setting is a string containing all plural variants, suitable
-      // for use in formatPlural() methods, but already translated into the
-      // UI language. Pop off the last plural variant, and use the parts
-      // before/after @count in that string as the prefix/suffix for this field.
+      // for use in plural formatting, translated into the UI language. Pop
+      // off the last plural variant, and use the parts before/after @count in
+      // that string as the prefix/suffix for this field.
       $values = explode(PluralTranslatableMarkup::DELIMITER, $field_settings['format_plural_string']);
       $labels = explode('@count', array_pop($values));
       if (isset($labels[0])) {
diff --git a/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php b/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
index fc7a85b..3d3b170 100644
--- a/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
+++ b/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
@@ -62,10 +62,10 @@ class PluralTranslatableMarkup extends TranslatableMarkup {
    *   (optional) An array with placeholder replacements, keyed by placeholder.
    *   See \Drupal\Component\Render\FormattableMarkup::placeholderFormat() for
    *   additional information about placeholders. Note that you do not need to
-   *   include @count in this array; this replacement is done automatically
-   *   for the plural cases. But you can include @count to override the
-   *   replacement value (for instance, to use a formatted number instead of
-   *   the raw number).
+   *   include @count in this array; this value is supplied automatically.
+   *   However, you can include a replacement for @count to override the
+   *   default of $count (for example, you might want to have a formatted
+   *   number instead of the raw number $count in the output).
    * @param array $options
    *   (optional) An associative array of additional options. See t() for
    *   allowed keys.
@@ -127,7 +127,9 @@ public function render() {
     }
 
     $arguments = $this->getArguments();
-    // Allow overrides of what gets substituted in for @count.
+    // Allow overrides of what gets substituted in for @count; for example, to
+    // include a formatted number in the output, while using the raw number to
+    // decide which plural form to use.
     if (!isset($arguments['@count'])) {
       $arguments['@count'] = $this->count;
     }
diff --git a/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php b/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php
index cfe8167..175a5a8 100644
--- a/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php
+++ b/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php
@@ -98,8 +98,10 @@ protected function getNumberOfPlurals($langcode = NULL) {
   protected function getPluralLabels($langcode) {
     $plurals = $this->getNumberOfPlurals($langcode);
 
-    // @todo The return values for both cases should be more relevant to the
-    // actual plural rules of the language; see
+    // @todo The return values should be made language-specific, so that they
+    // reflect the actual plural rules of languages. Also this method should
+    // be used in several forms that are currently calculating their own
+    // labels for plural variant forms. Issue:
     // https://www.drupal.org/node/2499639.
 
     if ($plurals == 1) {
@@ -112,6 +114,9 @@ protected function getPluralLabels($langcode) {
     }
 
     if ($plurals == 2) {
+      // @todo see above note. This text is good for typical 2-form languages,
+      // but it may need special cases for languages where the two forms do not
+      // correspond to 1 and != 1 values.
       return [
         [
           'label' => $this->t('Singular form'),
@@ -125,6 +130,9 @@ protected function getPluralLabels($langcode) {
     }
 
     // For the != 2 case, generate labels.
+    // @todo see above note. This definitely needs specific cases for languages
+    // that describe what the forms are used for, rather than saying
+    // Singular form, First plural form, 2. plural form, etc.
     $labels = [];
     for ($i = 0; $i < $plurals; $i++) {
       $labels[] = [