how can i change ?lang=en url to ?lang=swa when choosing language ?

i want to change a value that appeare in url when switchning to another language(local language/swahili) on my website

in the admin i choose two language to be used on the admin(english and french)
i’am using this code to display how public can choose a language

			#SET{tabLang,#CONFIG{langues_multilingue}|explode{","}}
			<B_lang>
			<ul>
			<BOUCLE_lang(DATA){source table,#GET{tabLang}}{si #GET{tabLang}|count|>{1}}>
			<li  id="LANG#COMPTEUR_BOUCLE"  class="[active(#LANG|=={#VALEUR}|oui)][ (#VALEUR)]" style="list-style:none;">
			<a href="[(#VAL{converser}|generer_url_action{redirect=#SELF}|parametre_url{var_lang,#VALEUR})]" rel="nofollow">[(#VALEUR|traduire_nom_langue)]</a>
			</li>
			</BOUCLE_lang>
			</ul>
			</B_lang>

and i want that if the visitor choose, for example, to navigate in english, the url change to look like

www.mydomain.com/?lang=swa
in stead of
www.mydomain.com/?lang=en

Note : i have chosen english file(/ecrire/lang/public_en.php) and modified it to my local language !

Thank you for your assistence