Start to simplify ::content slightly

src/Cms/ModelWithContent.php

@@ -132,21 +132,11 @@ public function content(string|null $languageCode = null): Content
 	{
 		// single language support
 		if ($this->kirby()->multilang() === false) {
-			if ($this->content instanceof Content) {
-				return $this->content;
-			}
-
-			// don't normalize field keys (already handled by the `Data` class)
-			return $this->content = new Content($this->readContent(), $this, false);
+			return $this->content ??= $this->version()->content('default');
 		}
 
 		// get the targeted language
-		$language = $this->kirby()->language($languageCode);
-
-		// stop if the language does not exist
-		if ($language === null) {
-			throw new InvalidArgumentException('Invalid language: ' . $languageCode);
-		}
+		$language = Language::ensure($languageCode);
 
 		// only fetch from cache for the current language
 		if ($languageCode === null && $this->content instanceof Content) {

tests/Cms/Models/ModelWithContentTest.php

@@ -6,6 +6,7 @@
 use Kirby\Content\Version;
 use Kirby\Content\VersionId;
 use Kirby\Exception\InvalidArgumentException;
+use Kirby\Exception\NotFoundException;
 use Kirby\Panel\Page as PanelPage;
 use Kirby\Uuid\PageUuid;
 use Kirby\Uuid\SiteUuid;
@@ -155,7 +156,7 @@ public function testContentForInvalidTranslation()
 			]
 		]);
 
-		$this->expectException(InvalidArgumentException::class);
+		$this->expectException(NotFoundException::class);
 		$this->expectExceptionMessage('Invalid language: fr');
 
 		$app->page('foo')->content('fr');