[html] Issue: Default <details> heading should be localized marked as i18n

chaals has just labeled an issue for https://github.com/w3c/html as 
"i18n":

== Default <details> heading should be localized ==
> If there is no child summary element, the user agent should provide 
its own legend (e.g. "Details").
> 
http://drafts.htmlwg.org/html/master/interactive-elements.html#the-details-element

In practice, UAs have implemented the default title of "Details", 
which does not seem to change depending on the specified language 
[[demo]](http://dabblet.com/gist/e0be732fbc828ebb5ea494edef1a5d4f). I 
think it should be explicitly mentioned that the default title should 
be localized.

See https://github.com/w3c/html/issues/470