The code actually works when it is a checkbox, i've changed the field type back to checkbox and if you take a look the field will auto select based on the url value.
In your example it's USA that is checked, but it should be Allemagne, since the offer's term is Allemagne, not USA !
You can verify this by checking the "offre" id in url parameter "id_offre", then check the taxonomy for this post id in backend.
What was selected is actually correct and is done based what is the value of that checkbox field. See Screenshot.
USA has a value of 7 and Allemagne has a value of 11 and the parameter value that was passed is 7 not 11. If 11 was passed then Allemagne would be selected.