Get Primary Country Language Code

Thanks a lot for your library, its very helpful.
It would be even more helpful if there was a new parameter `getLanguageCode` that could be used to get the primary language of a country as 2-letter (or even 5-letter) code:

```
// Get country language code
echo $austria->getLanguageCode();
```

// output
`'de'`
OR
`'de_AT'`

What do you think?