diff --git a/projects/plugins/wpcomsh/changelog/fix-theme-translation-files-loading b/projects/plugins/wpcomsh/changelog/fix-theme-translation-files-loading
new file mode 100644
index 00000000000..cad28c4c701
--- /dev/null
+++ b/projects/plugins/wpcomsh/changelog/fix-theme-translation-files-loading
@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+I18n: Correct WPCOM locale fallback when theme .mo files use {textdomain}-{locale} names under the themes directory.
diff --git a/projects/plugins/wpcomsh/i18n.php b/projects/plugins/wpcomsh/i18n.php
index 00fa2fe2266..0b0522b2b93 100644
--- a/projects/plugins/wpcomsh/i18n.php
+++ b/projects/plugins/wpcomsh/i18n.php
@@ -11,10 +11,11 @@
  *
  * @see p8yzl4-4c-p2
  *
- * @param string $mofile .mo language file being loaded by load_textdomain().
+ * @param string $mofile  .mo language file being loaded by load_textdomain().
+ * @param string $domain  Text domain.
  * @return string $mofile same or alternate mo file.
  */
-function wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile ) {
+function wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile, $domain = '' ) {
 	if ( file_exists( $mofile ) ) {
 		return $mofile;
 	}
@@ -27,8 +28,26 @@ function wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile ) {
 		require JETPACK__GLOTPRESS_LOCALES_PATH;
 	}
 
-	$locale_slug        = basename( $mofile, '.mo' );
-	$actual_locale_slug = $locale_slug;
+	$original_mo_basename = basename( $mofile, '.mo' );
+	$locale_slug          = $original_mo_basename;
+
+	$theme_root = function_exists( 'get_theme_root' )
+		? wp_normalize_path( trailingslashit( get_theme_root() ) )
+		: '';
+
+	// On WP Cloud sites, get_template_directory() returns /wordpress/themes/pub...
+	// which is the actual path to the theme, symlinked from wp-content/themes.
+	// _load_textdomain_just_in_time sees that the folders don't match
+	// and builds the mo file as "{$path}{$domain}-{$locale}.mo", so we need to
+	// remove the domain prefix to get the locale.
+	$remove_domain_prefix_for_theme = is_string( $domain ) && '' !== $domain
+		&& '' !== $theme_root
+		&& str_starts_with( wp_normalize_path( $mofile ), $theme_root )
+		&& str_starts_with( $original_mo_basename, $domain . '-' );
+
+	if ( $remove_domain_prefix_for_theme ) {
+		$locale_slug = str_replace( $domain . '-', '', $original_mo_basename );
+	}
 
 	// These locales are not in our GP_Locales file, so rewrite them.
 	$locale_mappings = array(
@@ -56,10 +75,10 @@ function wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile ) {
 	}
 
 	// phpcs:ignore WordPress.PHP.PregQuoteDelimiter.Missing
-	$mofile = preg_replace( '/' . preg_quote( $actual_locale_slug ) . '\.mo$/', $locale_slug . '.mo', $mofile );
+	$mofile = preg_replace( '/' . preg_quote( $original_mo_basename ) . '\.mo$/', $locale_slug . '.mo', $mofile );
 	return $mofile;
 }
-add_filter( 'load_textdomain_mofile', 'wpcomsh_wporg_to_wpcom_locale_mo_file', 9999 );
+add_filter( 'load_textdomain_mofile', 'wpcomsh_wporg_to_wpcom_locale_mo_file', 9999, 2 );
 
 // Load translations for wpcomsh itself via MO file.
 add_action(
diff --git a/projects/plugins/wpcomsh/tests/I18nTest.php b/projects/plugins/wpcomsh/tests/I18nTest.php
new file mode 100644
index 00000000000..9b92b250b84
--- /dev/null
+++ b/projects/plugins/wpcomsh/tests/I18nTest.php
@@ -0,0 +1,85 @@
+<?php
+/**
+ * Tests for i18n.php.
+ *
+ * @package wpcomsh
+ */
+
+/**
+ * Class I18nTest.
+ */
+class I18nTest extends WP_UnitTestCase {
+	use \Automattic\Jetpack\PHPUnit\WP_UnitTestCase_Fix;
+
+	/**
+	 * Ensure GlotPress locales are loadable (normally provided by Jetpack on Atomic).
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		if ( ! defined( 'JETPACK__GLOTPRESS_LOCALES_PATH' ) ) {
+			$locales = dirname( __DIR__, 3 ) . '/packages/compat/lib/locales.php';
+			if ( ! file_exists( $locales ) ) {
+				$this->markTestSkipped( 'Jetpack compat locales file not found: ' . $locales );
+			}
+			define( 'JETPACK__GLOTPRESS_LOCALES_PATH', $locales );
+		}
+	}
+
+	/**
+	 * When the .mo file already exists, the path must not be rewritten.
+	 */
+	public function test_returns_unchanged_when_mo_file_exists() {
+		$tmp = tempnam( sys_get_temp_dir(), 'wpcomsh-i18n-' );
+		$this->assertNotFalse( $tmp );
+		$mofile = $tmp . '.mo';
+		$this->assertTrue( rename( $tmp, $mofile ) );
+
+		$result = wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile, 'any-domain' );
+		$this->assertSame( $mofile, $result );
+
+		$this->assertTrue( unlink( $mofile ) );
+	}
+
+	public function test_rewrites_domain_prefixed_theme_mo_nb_no_to_no_slug() {
+		$theme_root = trailingslashit( wp_normalize_path( get_theme_root() ) );
+		$mofile     = $theme_root . 'faketheme/languages/faketheme-nb_NO.mo';
+		$this->assertFalse( file_exists( $mofile ) );
+
+		$result = wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile, 'faketheme' );
+
+		$this->assertSame( $theme_root . 'faketheme/languages/no.mo', $result );
+	}
+
+	public function test_rewrites_domain_prefixed_theme_mo_de_de_formal() {
+		$theme_root = trailingslashit( wp_normalize_path( get_theme_root() ) );
+		$mofile     = $theme_root . 'faketheme/languages/faketheme-de_DE_formal.mo';
+		$this->assertFalse( file_exists( $mofile ) );
+
+		$result = wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile, 'faketheme' );
+
+		$this->assertSame( $theme_root . 'faketheme/languages/de.mo', $result );
+	}
+
+	public function test_rewrites_unprefixed_theme_mo_de_de_to_de_slug() {
+		$theme_root = trailingslashit( wp_normalize_path( get_theme_root() ) );
+		$mofile     = $theme_root . 'faketheme/languages/de_DE.mo';
+		$this->assertFalse( file_exists( $mofile ) );
+
+		$result = wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile, 'faketheme' );
+
+		$this->assertSame( $theme_root . 'faketheme/languages/de.mo', $result );
+	}
+
+	/**
+	 * Paths outside get_theme_root() do not strip {domain}- from the basename; unknown wp_locale leaves path unchanged.
+	 */
+	public function test_does_not_rewrite_plugin_style_path_with_domain_like_basename() {
+		$mofile = trailingslashit( wp_normalize_path( WP_PLUGIN_DIR ) ) . 'some-plugin/languages/some-plugin-nb_NO.mo';
+		$this->assertFalse( file_exists( $mofile ) );
+
+		$result = wpcomsh_wporg_to_wpcom_locale_mo_file( $mofile, 'some-plugin' );
+
+		$this->assertSame( $mofile, $result );
+	}
+}