pb with urls

Hi
So here’s the blog I’m working on :
http://blog.1618-paris.com/
I don’t know if there is an incompability with my theme but it’s not working
I’m running the last version of the plugin, I would like a static front page

The problem is strange :
at this adress : http://blog.1618-paris.com, the adress bar will
– sometimes moove automatically to http://blog.1618-paris.com/?page_id=5 and the browser will display correctly the homepage
But if you click on “francais” on the right, you go to this adress : http://blog.1618-paris.com/?lang=fr and here everything in a mess, and the homepage slider doesn’t display
– or the contrary : you can arrive directly in french, it’s a mess

In the french mess, if you’re lucky enough you’ll find, behind the “MI” of “MINIMUM”, the home button, that will this time leads you to : http://blog.1618-paris.com/en/
who just doesn’t exist…
By chance other pages work, but maybe because they’re mostly in english…
So I don’t know, I’m quite desperate

NB : I wanted to change my settings, but some of them are grey, can’t change anything…
FOr example, in the URL modifications :
it is already set on :
– The language is set from content. Posts, pages, categories and tags urls are not modified.
And I can’t change it to :
– The language code, for example /en/, is added to all urls when using pretty permalinks.

What should I do? Renouncing to such a great plugin?

http://wordpress.org/extend/plugins/polylang/