Fallback language

Hello,
is there a way to set a fallback language?
for example: 

_en.json_

``` json
{
  "message": {
    "hello": "Hi"
  } 
}
```

_spanish.json_

``` json
{
  "message": { 
  } 
}
```

``` javascript
Vue.config.lang = 'spanish'
Vue.config.fallbackLang = 'en'

$t('message.hello')
// would return `Hi` 

```

Is there a way to do this ?
