Is it to possible to insert a page in the post depending on the category ofthe post

Yup, inside the loop (in single.php for example) you can use get_the_terms to retrieve an array of terms (countries in this case) associated with that post. This will always be an array (or false /error if something has gone wrong) even if there is only one country associated to it.

You can then take the country taxonomy-term and output its description use term_description.

$taxonomy = 'my-country-tax'; Your taxonomy slug

//Should be an array of 1 in this case
$countries = get_the_terms( $post->ID, $taxonomy);

//Check there are countries asccoiated with post
if($countries && !is_wp_error($countries)){
    $country = array_pop($countries);
    $description= term_description( $country->term_id, $taxonomy );

echo $description;
}