Fix issue with the Admin Language switcher

Closes #18

src/Loader.php

@@ -16,10 +16,32 @@ function bind_hooks()
     {
         add_filter('pll_model', [$this, '__filter_pll_model'], 10, 1);
         add_filter('pll_context', [$this, '__filter_pll_context'], 10, 1);
+        add_filter(
+            'get_user_metadata',
+            [$this, '__filter_get_user_metadata'],
+            10,
+            4
+        );
         add_action('graphql_init', [$this, '__action_graphql_init']);
         add_action('admin_notices', [$this, '__action_admin_notices']);
     }
 
+    /**
+     * Disable wp-admin language switcher on graphql context
+     */
+    function __filter_get_user_metadata($data, $object_id, $meta_key, $single)
+    {
+        if ($meta_key !== 'pll_filter_content') {
+            return $data;
+        }
+
+        if (!$this->is_graphql_request()) {
+            return $data;
+        }
+
+        return 'all';
+    }
+
     function __action_graphql_init()
     {
         if (!$this->is_graphql_request()) {

tests/wpunit/PostObjectQueryTest.php

@@ -241,4 +241,34 @@ public function testContentNodesFiltering()
 
         $this->assertEquals($nodes, $expected);
     }
+
+    public function testListsPostsFromAllLanguagesWhenLanguageIsSelectedInWPAdmin()
+    {
+        wp_set_current_user(1);
+        update_user_meta(1, 'pll_filter_content', 'en');
+
+        // XXX This is not enough to initialize the admin mode for Polylang
+        // set_current_screen( 'edit-post' );
+        set_current_screen('edit.php');
+
+        $query = '
+        query Posts {
+            posts {
+              nodes {
+                title
+              }
+            }
+         }
+        ';
+
+        $data = do_graphql_request($query);
+        $nodes = $data['data']['posts']['nodes'];
+        $expected = [
+            //
+            ['title' => 'English post'],
+            ['title' => 'Finnish post'],
+        ];
+
+        $this->assertEquals($nodes, $expected);
+    }
 }