Add function for determining whether an object is localized

If a property is localised, we get back an object that looks like this...

```
{
  en: 'English text',
  fr: 'French text,
  fr-CA: 'French Canadian text'
}
```

In order to know whether an object is truly a localised object, we need to validate that all of the keys are valid locales.

We need a function that encapsulates this piece of logic.