Permalinks don't match where files for non-default language are placed

[rgommers]

When using permalink: /:title.html the posts end up in the right place (in _site/postname.html for default lang and in _site/lang/postname.html for other languages). Permalinks for the default language are also correct, but for other languages post.url misses /lang. For other formats of permalink the issue will be the same.

I don't see a bug report for this yet, although gh-18 seems to be related. What would be the best way to fix this?