Add cross-language links in the index template.

[JulienPalard]

I worked on: #203

But I'm not happy at all with my UI/UX skills.

I even had to ask a friend about how to "compile" the ".less" file :D

For reference, it's how it already looks like in an article page, I did not changed nothing here:

My PR add it to the indexes pages, it looks like:

So technically, it works, visually it's ... unacceptable :D

[alexandrevicenzi]

I'm working on some improvements, and this currently does not look that good.

If you don't mind, I will close this, and fix it properly on Flex v3.