fix language selector (dotnet#36)

Author: mairaw

docfx.json

@@ -40,9 +40,6 @@
     "externalReference": [],
     "globalMetadata": {
       "breadcrumb_path": "/dotnet/breadcrumb/toc.json",
-      "_displayLangs": [
-        "csharp"
-      ],
       "author": "dotnet-bot",
       "ms.author": "dotnetcontent",
       "manager": "wpickett",
@@ -62,6 +59,10 @@
       "ms.topic": {
         "api/**.yml": "managed-reference"
      },
+      "langs": {
+        "api/**.yml": ["csharp", "vb", "cpp"],
+        "api/System.Convert.yml": ["csharp", "vb", "cpp", "fsharp"]        
+      },
       "ROBOTS": {
         "api/**.yml": "NOINDEX, NOFOLLOW"
       }