diff --git a/classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php b/classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php
new file mode 100644
index 0000000000..f6c5ff6a6a
--- /dev/null
+++ b/classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php
@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * @file classes/migration/upgrade/v3_4_0/MergeLocalesMigration.php
+ *
+ * Copyright (c) 2014-2021 Simon Fraser University
+ * Copyright (c) 2000-2021 John Willinsky
+ * Distributed under the GNU GPL v3. For full terms see the file docs/COPYING.
+ *
+ * @class MergeLocalesMigration
+ *
+ * @brief Change Locales from locale_countryCode localization folder notation to locale localization folder notation
+ */
+
+namespace APP\migration\upgrade\v3_4_0;
+
+use Illuminate\Support\Collection;
+use Illuminate\Support\Facades\DB;
+use PKP\install\DowngradeNotSupportedException;
+
+class MergeLocalesMigration extends \PKP\migration\upgrade\v3_4_0\MergeLocalesMigration
+{
+    protected string $CONTEXT_TABLE = 'servers';
+    protected string $CONTEXT_SETTINGS_TABLE = 'server_settings';
+    protected string $CONTEXT_COLUMN = 'server_id';
+
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        parent::up();
+
+        // publication_galleys
+        $publicationGalleys = DB::table('publication_galleys')
+            ->get();
+
+        foreach ($publicationGalleys as $publicationGalley) {
+            $this->updateSingleValueLocale($publicationGalley->locale, 'publication_galleys', 'locale', 'galley_id', $publicationGalley->galley_id);
+        }
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        throw new DowngradeNotSupportedException();
+    }
+
+    protected function getSettingsTables(): Collection
+    {
+        return collect([
+            'server_settings' => 'server_id',
+            'publication_galley_settings' => 'galley_id',
+            'section_settings' => 'section_id',
+        ])->merge(parent::getSettingsTables());
+    }
+}
diff --git a/config.TEMPLATE.inc.php b/config.TEMPLATE.inc.php
index a38a96a1c2..9bbb75b62b 100644
--- a/config.TEMPLATE.inc.php
+++ b/config.TEMPLATE.inc.php
@@ -204,7 +204,7 @@
 [i18n]
 
 ; Default locale
-locale = en_US
+locale = en
 
 ; Database connection character set
 connection_charset = utf8
diff --git a/dbscripts/xml/upgrade.xml b/dbscripts/xml/upgrade.xml
index 70e3243609..3a6109034a 100644
--- a/dbscripts/xml/upgrade.xml
+++ b/dbscripts/xml/upgrade.xml
@@ -84,6 +84,7 @@
 		<migration class="APP\migration\upgrade\v3_4_0\I7513_DoiSettings"/>
 		<migration class="PKP\migration\upgrade\v3_4_0\I7486_RenameUnconsideredColumnToConsidered"/>
 		<migration class="PKP\migration\upgrade\v3_4_0\I7486_RemoveItemViewsTable"/>
+		<migration class="APP\migration\upgrade\v3_4_0\MergeLocalesMigration"/>
 		<note file="docs/release-notes/README-3.4.0" />
 	</upgrade>