allow json values for each language

Removing string type hints allows using Translatable with array values,
e.g.:

['en' => ['subkey' => 'foo']]

src/Translatable.php

@@ -20,7 +20,7 @@ public function getAttribute($key)
         return parent::getAttribute($key);
     }
 
-    public function getValueOfCurrentLocaleForKey(string $key): ?string
+    public function getValueOfCurrentLocaleForKey(string $key)
     {
         $translations = $this->getAttributeValue($key);
 

src/TranslationsManager.php

@@ -11,7 +11,7 @@ public function __construct(?array $translations)
         $this->translations = $translations;
     }
 
-    public function __get(string $key): ?string
+    public function __get(string $key)
     {
         return $this->translations[$key] ?? null;
     }