Update index.html

HTML is more W3C and XML compliant.

More flexible way to manage lang
A way to define the navigator langage and apply it to the page
I have not done a lot of JS, so it might not work. Moreover, It would be more optimized to search the default option langage only if it has not yet be defined with navigator.langage.
Merge pull request #42 from RyDroid/patch-1

Update index.html