This isn't going to work like you expect.

If you test in the console, it's possible to call `slice(0)` on an empty array.  I would do:

```javascript
listSupportedLang = options.supported_languages && options.supported_languages.length && options.supported_languages.slice(0) || ['en-US'];
```

or the equivalent logic with if blocks:

```javascript
if ( options.supported_languages && options.supported_languages.length ) {
  listSupportedLang =  options.supported_languages.slice(0);
} else {
  listSupportedLang = ['en-US'];
```

Both should work fine.