PW-2068 Change shopper locale config input type (#676)

Co-Authored-By: Ángel Campos <angel.campos@adyen.com>

Model/Config/Source/InheritableLocale.php

+<?php
+
+namespace Adyen\Payment\Model\Config\Source;
+
+use Magento\Config\Model\Config\Source\Locale as MagentoLocale;
+
+class InheritableLocale extends MagentoLocale
+{
+    public function toOptionArray()
+    {
+        $locales = parent::toOptionArray();
+        array_unshift($locales, [
+            'value' => '',
+            'label' => 'Use system locale'
+        ]);
+        return $locales;
+    }
+}
\ No newline at end of file

etc/adminhtml/system/main.xml

@@ -28,8 +28,9 @@
         <source_model>Adyen\Payment\Model\Config\Source\DemoMode</source_model>
         <config_path>payment/adyen_abstract/demo_mode</config_path>
     </field>
-    <field id="shopper_locale" translate="label" type="text" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="1">
+    <field id="shopper_locale" translate="label" type="select" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="1">
         <label>Locale</label>
+        <source_model>Adyen\Payment\Model\Config\Source\InheritableLocale</source_model>
         <config_path>payment/adyen_hpp/shopper_locale</config_path>
     </field>
     <field id="version" translate="label" type="label" sortOrder="40" showInDefault="1" showInWebsite="1" showInStore="0">