diff --git a/src/wp-admin/install.php b/src/wp-admin/install.php
index bf102893ff..34d5d4c7f7 100644
--- a/src/wp-admin/install.php
+++ b/src/wp-admin/install.php
@@ -329,7 +329,7 @@ if ( defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) {
  */
 $language = '';
 if ( ! empty( $_REQUEST['language'] ) ) {
-	$language = preg_replace( '/[^a-zA-Z0-9_]/', '', $_REQUEST['language'] );
+	$language = sanitize_locale_name( $_REQUEST['language'] );
 } elseif ( isset( $GLOBALS['wp_local_package'] ) ) {
 	$language = $GLOBALS['wp_local_package'];
 }
diff --git a/src/wp-includes/l10n.php b/src/wp-includes/l10n.php
index 67bfb1a26e..598a485468 100644
--- a/src/wp-includes/l10n.php
+++ b/src/wp-includes/l10n.php
@@ -150,6 +150,15 @@ function determine_locale() {
 		( isset( $_GET['_locale'] ) && 'user' === $_GET['_locale'] && wp_is_json_request() )
 	) {
 		$determined_locale = get_user_locale();
+	} elseif (
+		( ! empty( $_REQUEST['language'] ) || isset( $GLOBALS['wp_local_package'] ) )
+		&& wp_installing()
+	) {
+		if ( ! empty( $_REQUEST['language'] ) ) {
+			$determined_locale = sanitize_locale_name( $_REQUEST['language'] );
+		} else {
+			$determined_locale = $GLOBALS['wp_local_package'];
+		}
 	}
 
 	if ( ! $determined_locale ) {
diff --git a/tests/phpunit/tests/l10n/determineLocale.php b/tests/phpunit/tests/l10n/determineLocale.php
index 51ab1c28b9..3a675dc515 100644
--- a/tests/phpunit/tests/l10n/determineLocale.php
+++ b/tests/phpunit/tests/l10n/determineLocale.php
@@ -20,7 +20,15 @@ class Tests_L10n_DetermineLocale extends WP_UnitTestCase {
 	}
 
 	public function tear_down() {
-		unset( $_SERVER['CONTENT_TYPE'], $_GET['_locale'], $_COOKIE['wp_lang'], $GLOBALS['pagenow'] );
+		unset(
+			$_SERVER['CONTENT_TYPE'],
+			$_GET['_locale'],
+			$_COOKIE['wp_lang'],
+			$GLOBALS['pagenow'],
+			$GLOBALS['wp_local_package'],
+			$_REQUEST['language']
+		);
+		wp_installing( false );
 
 		parent::tear_down();
 	}
@@ -273,4 +281,31 @@ class Tests_L10n_DetermineLocale extends WP_UnitTestCase {
 
 		$this->assertSame( 'siteLocale', determine_locale() );
 	}
+
+	public function test_language_param_not_installing() {
+		$_REQUEST['language'] = 'de_DE';
+		$this->assertSame( 'en_US', determine_locale() );
+	}
+
+	public function test_language_param_installing() {
+		$_REQUEST['language'] = 'de_DE';
+		wp_installing( true );
+		$this->assertSame( 'de_DE', determine_locale() );
+	}
+
+	public function test_language_param_installing_incorrect_string() {
+		$_REQUEST['language'] = '####';  // Something sanitize_locale_name() strips away.
+		wp_installing( true );
+		$this->assertSame( 'en_US', determine_locale() );
+	}
+
+	public function test_wp_local_package_global_not_installing() {
+		$_REQUEST['language'] = 'de_DE';
+		$this->assertSame( 'en_US', determine_locale() );
+	}
+	public function test_wp_local_package_global_installing() {
+		$_REQUEST['language'] = 'de_DE';
+		wp_installing( true );
+		$this->assertSame( 'de_DE', determine_locale() );
+	}
 }