Excellent job, I tested it out and it works great.
I did find one typo, I underlined it for ya.
Code:
public plugin_init(){
register_plugin("admin alltalk","0.1","GORT")
register_concmd("amx_alltalk","admin_alltalk",ADMIN_KICK,"< 0/1 > <u><b>trun</b></u> sv_alltalk on or off")
register_clcmd("say /alltalk","check_alltalk")
register_clcmd("say_team /alltalk","check_alltalk")
return PLUGIN_CONTINUE