Implement default translation content fallback

src/Content/Version.php

@@ -33,9 +33,25 @@ public function __construct(
 	 */
 	public function content(Language|string $language = 'default'): Content
 	{
+		$language = Language::ensure($language);
+		$fields   = $this->read($language);
+
+		// This is where we merge content from the default language
+		// to provide a fallback for missing/untranslated fields.
+		//
+		// @todo This is the critical point that needs to be removed/refactored
+		// in the future, to provide multi-language support with truly
+		// individual versions of pages and no longer enforce the fallback.
+		if ($language->isDefault() === false) {
+			$fields = [
+				...$this->read('default'),
+				...$fields
+			];
+		}
+
 		return new Content(
 			parent: $this->model,
-			data:   $this->read($language),
+			data:   $fields,
 		);
 	}
 

tests/Content/VersionTest.php

@@ -111,6 +111,31 @@ public function testContentSingleLanguage(): void
 		$this->assertSame($expected['content']['title'], $version->content()->get('title')->value());
 	}
 
+	/**
+	 * @covers ::content
+	 */
+	public function testContentWithFallback(): void
+	{
+		$this->setUpMultiLanguage();
+
+		$version = new Version(
+			model: $this->model,
+			id: VersionId::published()
+		);
+
+		// write something to the content file to make sure it
+		// can be read from disk for the test.
+		Data::write($this->model->root() . '/article.en.txt', $content = [
+			'title' => 'Test'
+		]);
+
+		$this->assertSame($content, $version->content()->toArray());
+		$this->assertSame($content, $version->content('en')->toArray());
+
+		// make sure that the content fallback works
+		$this->assertSame($version->content('en')->toArray(), $version->content('de')->toArray());
+	}
+
 	/**
 	 * @covers ::contentFile
 	 */