post_status_form: inherit language from parent

If I'm replying to a post in Klingon, chances are I'm going to write in
Klingon. This reduces friction for properly marking post language in a
conversation.

src/components/post_status_form/post_status_form.js

@@ -180,7 +180,7 @@ const PostStatusForm = {
         poll: this.statusPoll || {},
         mediaDescriptions: this.statusMediaDescriptions || {},
         visibility: this.statusScope || this.suggestedVisibility(),
-        language: this.statusLanguage || postLanguage,
+        language: this.statusLanguage || this.suggestedLanguage(),
         contentType: statusContentType
       }
     }