No it is not. You can edit the values of cookies like of GET or POST Parameter.
Firebug is able to do this in Firefox and Chrome got the native Debugger in this, so also validate the content of your cookie.
This is a common mistake:
if(isset($_COOKIE['lang'])){
$lang=$_COOKIE['lang'];
}else{
$lang="english";
}
include ("translation/".$lang.".html");
This would allow in circumstances a Local File Inclusion (LFI), since the content of $_COOKIE['lang'] can be modified.