Language codes

Adding language code to the api to consume and cache in consumer domain 
There could be more validated here - this method will throw a NoMethodError for nil when response is nil. I would check these things to prevent all of the nil response errors:
```ruby
required_response_field(root_node, response)
required_response_field(collection_node, response[root_node])
required_response_field('LanguageCode', response[root_node][collection_node])
```
Doing all 3 of those will ensure that when you call `model_hash`, there are no 'NoMethodError :[] for nil' (hash accessor method called on nil) errors.