As I edit my files with [vim](http://vim.org), it is really easy to add `fr:` or `en:` at some line's beginning using the useful `C-v`.
However [nanoc](http://nanoc.stoneship.org) was conceived to be used for one language only. Or to be used with the first method. I tried to adapt nanoc to my usage. But after a while, I found it easier to pre-filter the nanoc work by a simple script. My script transform my file into two new files. And all work like a charm.