Alter a select form using preprocess function
In your foreach loop, $option_term is a copy of the current variable returned from the iteration. If you want to change the original variable (i.e. what's in $vars['form']['field_categorie_tid']['#options'][index]) then you need to pass it by reference:

foreach($vars['form']['field_categorie_tid']["#options"] as &amp;$option_term){
    $option_term .= 'test';
}


Or refer to the array member explicitly like so:

foreach($vars['form']['field_categorie_tid']["#options"] as $key =&gt; $option_term){
    $vars['form']['field_categorie_tid']["#options"][$key] .= 'test';
}


I generally prefer the second method as passing variables by reference in this way can sometimes cause some strange behaviour.

You also had a slight code error, the variable is called $option_term in your loop declaration but $optionterm in the loop body.

Just FYI, template preprocess functions are really not the best place for this sort of code. You would be far better off using hook_form_alter().