Default language

The theme I'm using has 3 files in the languages folder , `default`, `fr-FR` and `zh-Hans`.
I need English (default) as my site language. Leaving the `language` option in `_config.yml` blank doesn't fallback to default (as I suppose is the expected behavior). iissnan/hexo-theme-next#57 has more details.
In the line [lib/theme/index.js#L26](https://github.com/hexojs/hexo/blob/master/lib/theme/index.js#L26) it isn't checked if `languages` is `null` before adding it to the array. Maybe the bug is here.