diff --git a/source/wp-content/themes/wporg-parent-2021/functions.php b/source/wp-content/themes/wporg-parent-2021/functions.php
index 95f83f0e..71a516c8 100644
--- a/source/wp-content/themes/wporg-parent-2021/functions.php
+++ b/source/wp-content/themes/wporg-parent-2021/functions.php
@@ -8,6 +8,7 @@
 
 require_once __DIR__ . '/inc/gutenberg-tweaks.php';
 require_once __DIR__ . '/inc/block-styles.php';
+require_once __DIR__ . '/inc/rosetta-styles.php';
 
 /**
  * Actions and filters.
diff --git a/source/wp-content/themes/wporg-parent-2021/inc/rosetta-styles.php b/source/wp-content/themes/wporg-parent-2021/inc/rosetta-styles.php
new file mode 100644
index 00000000..f154481a
--- /dev/null
+++ b/source/wp-content/themes/wporg-parent-2021/inc/rosetta-styles.php
@@ -0,0 +1,92 @@
+<?php
+/**
+ * Rosetta customizations.
+ */
+
+namespace WordPressdotorg\Theme\Parent_2021\Rosetta_Styles;
+
+defined( 'WPINC' ) || die();
+
+add_filter( 'wp_theme_json_data_user', __NAMESPACE__ . '\inject_i18n_customizations' );
+
+/**
+ * Inject customizations for Rosetta sites.
+ *
+ * @param WP_Theme_JSON_Data $theme_json Class to access and update the underlying data.
+ *
+ * @return WP_Theme_JSON_Data The updated user settings.
+ */
+function inject_i18n_customizations( $theme_json ) {
+	$locale_settings = get_locale_customizations( get_locale() );
+	if ( ! $locale_settings ) {
+		return $theme_json;
+	}
+
+	$config = array(
+		'version' => 2,
+		'settings' => $locale_settings,
+	);
+
+	return new \WP_Theme_JSON( $config, 'custom' );
+}
+
+/**
+ * Get a theme.json-shaped array with custom values for a given locale.
+ *
+ * The returned array should match the structure of "settings" in a theme.json
+ * file. These will be loaded as the "user" settings, which will override the
+ * theme.json values. Rosetta sites can then override any of the generated
+ * custom properties (ex, --wp--preset--font-size--normal) in a way that will
+ * cascade to any future child themes, and also render correctly in the editor.
+ *
+ * @param string $locale The current site locale.
+ *
+ * @return array An array of settings mirroring a theme.json "settings" object.
+ */
+function get_locale_customizations( $locale ) {
+	switch ( $locale ) {
+		case 'ca':
+		case 'fr':
+		case 'it_IT':
+		case 'ro_RO':
+			return [
+				'typography' => [
+					'fontSizes' => [
+						[
+							'slug' => 'heading-cta',
+							'size' => '96px',
+						],
+					],
+				],
+			];
+		case 'ja':
+			return [
+				'custom' => [
+					'heading' => [
+						'cta' => [
+							'breakpoint' => [
+								'small-only' => [
+									'typography' => [
+										'fontSize' => '50px',
+									],
+								],
+							],
+						],
+					],
+				],
+				'typography' => [
+					'fontSizes' => [
+						[
+							'slug' => 'heading-cta',
+							'size' => '96px',
+						],
+						[
+							'slug' => 'heading-2',
+							'size' => '40px',
+						],
+					],
+				],
+			];
+	}
+	return false;
+}