Redirect language URLs with non-translated slugs

Fixes #3550

src/Cms/LanguageRoutes.php

@@ -3,6 +3,7 @@
 namespace Kirby\Cms;
 
 use Kirby\Filesystem\F;
+use Kirby\Toolkit\Str;
 
 class LanguageRoutes
 {
@@ -29,9 +30,26 @@ public static function create(App $kirby): array
 				'pattern' => $language->pattern(),
 				'method'  => 'ALL',
 				'env'     => 'site',
-				'action'  => function ($path = null) use ($language) {
+				'action'  => function ($path = null) use ($kirby, $language) {
 					$result = $language->router()->call($path);
 
+					// redirect secondary-language pages that have
+					// been accessed with non-translated slugs in their path
+					// to their fully translated URL
+					if ($path !== null && $result instanceof Page) {
+						if (Str::endsWith($result->url(), $path) === false) {
+							$url   = $result->url();
+							$query = $kirby->request()->query()->toString();
+
+							// preserve query across redirect
+							if (empty($query) === false) {
+								$url .= '?' . $query;
+							}
+
+							return Response::redirect($url);
+						}
+					}
+
 					// explicitly test for null as $result can
 					// contain falsy values that should still be returned
 					if ($result !== null) {

tests/Cms/Languages/LanguageRoutesTest.php

@@ -122,4 +122,42 @@ public function testNotNextWhenFalsyReturn()
 		$this->assertSame(1, $d);
 		$this->assertSame(2, $e);
 	}
+
+	public function testRedirectWhenNonTranslatedSlugs()
+	{
+		$app = $this->app->clone([
+			'site' => [
+				'children' => [
+					[
+						'slug'         => 'page1',
+						'translations' => [
+							[
+								'code' => 'en',
+							],
+							[
+								'code' => 'de',
+								'slug' => 'seite1',
+							]
+						]
+					]
+				]
+			],
+			'request' => [
+				'query' => [
+					'foo' => 'bar',
+				]
+			]
+		]);
+
+		$result = $app->call('page1');
+		$this->assertSame($app->page('page1'), $result);
+
+		$result = $app->call('de/page1');
+		$this->assertInstanceOf(Response::class, $result);
+		$this->assertSame(302, $result->code());
+		$this->assertSame('/de/seite1?foo=bar', $result->header('Location'));
+
+		$result = $app->call('de/seite1');
+		$this->assertSame($app->page('page1'), $result);
+	}
 }

tests/Uuid/PageUuidTest.php

@@ -156,7 +156,6 @@ public function testMultilang(string $language, string $title)
 							],
 							[
 								'code' => 'de',
-								'slug' => 'bar',
 								'content' => [
 									'title' => 'Bar',
 								]