[5.x] Add optional fallback for missing keys in {{ trans }} tag (statamic#11944)

Author: daun

src/Tags/Trans.php

@@ -13,8 +13,14 @@ public function wildcard($tag)
     {
         $key = $this->params->get('key', $tag);
         $locale = $this->params->pull('locale') ?? $this->params->pull('site');
+        $fallback = $this->params->get('fallback');
         $params = $this->params->all();
 
-        return __($key, $params, $locale);
+        $translation = __($key, $params, $locale);
+        if ($fallback && $translation === $key) {
+            return __($fallback, $params, $locale);
+        } else {
+            return $translation;
+        }
     }
 }

tests/Tags/TransTagTest.php

@@ -38,4 +38,18 @@ public function it_translates_to_specific_locale()
         $this->assertEquals('Bonjour, Bob', $this->parse('{{ trans key="package::messages.hello_name" name="Bob" locale="fr" }}'));
         $this->assertEquals('Bonjour, Bob', $this->parse('{{ trans key="package::messages.hello_name" name="Bob" site="fr" }}'));
     }
+
+    #[Test]
+    public function it_falls_back_to_fallback_for_missing_key()
+    {
+        $this->assertEquals('Fallback', $this->parse('{{ trans key="package::messages.does_not_exist" fallback="Fallback" }}'));
+        $this->assertEquals('Bonjour', $this->parse('{{ trans key="package::messages.does_not_exist" fallback="package::messages.hello" locale="fr" }}'));
+    }
+
+    #[Test]
+    public function it_applies_replacement_to_fallbacks()
+    {
+        $this->assertEquals('Fallback Bob', $this->parse('{{ trans key="package::messages.does_not_exist" name="Bob" fallback="Fallback :name" }}'));
+        $this->assertEquals('Bonjour, Bob', $this->parse('{{ trans key="package::messages.does_not_exist" name="Bob" fallback="package::messages.hello_name" locale="fr" }}'));
+    }
 }