Docs: How to set fallback locale?

When a message is not found, how to set fallback locale? For example, I would like to set the fallback locale to be 'de', but currently it seems to be hardcoded to en:

```
[intlify] Not found 'hello' key in 'es' locale messages.
[intlify] Fall back to translate 'hello' key with 'en-US' locale.
[intlify] Fall back to translate 'hello' key with 'en' locale.
```