fix(L10N): stop stripping _ from language codes

Stripping the underscore breaks support for all languages like de_AT,
de_DE and so on...

Signed-off-by: Salvatore Martire <4652631+salmart-dev@users.noreply.github.com>

lib/private/L10N/Factory.php

@@ -145,7 +145,7 @@ class Factory implements IFactory {
 		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);
 	}
 

tests/lib/L10N/FactoryTest.php

@@ -95,6 +95,7 @@ class FactoryTest extends TestCase {
 		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'],
 		];