à la place de ci- dessu essayer sa :
{if $category->id == 1 OR $nb_products == 0}
<span>{l s='There are no products in this category'}</span>
{else}
{if $nb_products == 1}
(<span>{l s='%d ' sprintf=$nb_products}</span>{l s='Item' })
{else}
(<span>{l s='%d ' sprintf=$nb_products}</span>{l s='Items' })
{/if}
{/if}