Hi,
for the your backend access i tried (with your Keys) and it works. Maybe you are gone on my real site and not on the test site. On the test site, sometimes, there is some link that goes on the real site.
Regarding the frontend error i made a test with your username and added another article so now i have this:
There are 4 error message.
I think that maybe you (Nicolas) should see the 4 messages because are the master but the customer have to see just the first one because he has only one way to complete the order: to buy over the minimum price allowed.
For the translation, yes, i have not the file translated but, for a test site, never mind.
PS: I sent you the access for the backend; maybe you have a wrong access