diff --git a/lib/private/L10N/Factory.php b/lib/private/L10N/Factory.php
index 993add47781b2..9a8c9e690f88c 100644
--- a/lib/private/L10N/Factory.php
+++ b/lib/private/L10N/Factory.php
@@ -146,7 +146,7 @@ private function cleanLanguage(?string $lang): ?string {
 		if ($lang === null) {
 			return null;
 		}
-		$lang = preg_replace('/[^a-zA-Z0-9.;,=-]/', '', $lang);
+		$lang = preg_replace('/[^a-zA-Z0-9.;,=_-]/', '', $lang);
 		return str_replace('..', '', $lang);
 	}
 
diff --git a/tests/lib/L10N/FactoryTest.php b/tests/lib/L10N/FactoryTest.php
index 9df113dee36e5..1084558bceb54 100644
--- a/tests/lib/L10N/FactoryTest.php
+++ b/tests/lib/L10N/FactoryTest.php
@@ -94,6 +94,7 @@ public static function dataCleanLanguage(): array {
 		return [
 			'null shortcut' => [null, null],
 			'default language' => ['de', 'de'],
+			'regional language' => ['de_DE', 'de_DE'],
 			'malicious language' => ['de/../fr', 'defr'],
 			'request language' => ['kab;q=0.8,ka;q=0.7,de;q=0.6', 'kab;q=0.8,ka;q=0.7,de;q=0.6'],
 		];