The default configuration before I18n is loaded is not working

Because `"use strict"` is called at the first place of the function, `this` in the function will be `undefined` and then the default configuration below is not working.

```js
I18n = {} // You must define this object in top namespace, which should be `window`
I18n.defaultLocale = "pt-BR";
I18n.locale = "pt-BR";

// Load I18n from `i18n.js`, `application.js` or whatever

I18n.currentLocale();
// pt-BR
```

I fixed by passing the global object to the function