Feature request: Translatable aria-label for the close button

[NatalieMac]

• Feature Request. Make sure there's no good way to do what you want first;
  consider asking on http://stackoverflow.com/questions/ask first.

The 'Close' button gets an aria-label with 'Close' as the value, but it would be lovely to be able to set that value in the configuration for cases where 'Close' needs to be translated.