WordPress MU

hello. i want to modify wpmu's home page. i want language of it is selectable by guest user with GET variable and/or select language from cookie. should i modify core files or can i make separate plug-in? if i must modify then where i should?

now i have found function get_locale() { ... in /wp-includes/l10n.php .

i have found that apply_filters function that is called in get_locale function overwrites $locale variable with "correct" (that is in settings and/or in database).
can i do not call apply_filters ? i checked and have known out that returning $locale (it is like 'en_GB') directly (without passing it through "apply_filters" function) makes the blog change language.

in http://mu.wordpress.org/forums/topic.php?id=2042&page=2#post-67728 "Multi language set up" i have just now said:
"now i have home page quickly can change language this way. i have added "?l=en" like links to multilingual home theme. current language is saved in cookie.
...
i know now that i probably can change $locale with apply_filters."

function get_locale() {
	global $locale;
	//mod
	//error_log($_COOKIE['forum_guest_language']."\r\n",3,'debuglog.txt');
	//endmod
	//mod
	//error_log($locale."\r\n",3,'debuglog.txt');
	//error_log(apply_filters( 'locale', $locale )."\r\n",3,'debuglog.txt');
	//error_log((isset($locale)?'1':'0')."\r\n",3,'debuglog.txt');
	//error_log((isset($locale)?'1':'0').':'.$locale.':'.apply_filters('locale',$locale)."\r\n",3,'debuglog.txt');
	//endmod
	//mod
	if($_SERVER['HTTP_HOST']=='wp.domain.tld'&&!headers_sent()){
		if($_GET['l']=='l1'){
			$locale='l1_CO';
			setcookie('forum_guest_language','Lang1',time()+365*24*60*60,'/','domain.tld');
		}elseif($_GET['l']=='l2'){
			$locale='l2_CO';
			setcookie('forum_guest_language','Lang2',time()+365*24*60*60,'/','domain.tld');
		}else{
			$cookie_guest_language=$_COOKIE['forum_guest_language'];
			if($cookie_guest_language=='Lang1'){$locale='l1_CO';}
			elseif($cookie_guest_language=='Lang2'){$locale='l2_CO';}
			else{$locale='l1_CO';}
		}
		return $locale;
	}else{
		if (isset($locale))
			return apply_filters( 'locale', $locale );
	}
	//endmod
	//mod
	//if($_GET['l']=='l1'){$locale='l1_CO';}elseif($_GET['l']=='l2'){$locale='l2_CO';}else{$locale='l1_CO';}
	//error_log(apply_filters( 'locale', $locale )."\r\n",3,'debuglog.txt');
	//error_log($_GET['l']."\r\n",3,'debuglog.txt');
	//error_log($locale."\r\n",3,'debuglog.txt');
	//return 'l2_CO';
	//endmod

	//if (isset($locale))
	//	return apply_filters( 'locale', $locale );
	//mod
	//if (isset($locale)){
	//	error_log($locale.':',3,'debuglog.txt');
	//	$qwert=apply_filters( 'locale', $locale );
	//	//$qwert=apply_filters( 'locale', '43534' );
	//	error_log($qwert."\r\n",3,'debuglog.txt');
	//	return $qwert;
	//}
	//endmod
	//mod
	//return $locale;
	//endmod
	//mod
	//error_log('$localewasnotset',3,'debuglog.txt');
	//endmod

	// WPLANG is defined in wp-config.
	if (defined('WPLANG'))
		$locale = WPLANG;
	//mod
	//error_log((isset($locale)?'1':'0').':'.$locale.':'.apply_filters('locale',$locale)."\r\n",3,'debuglog.txt');
	//error_log((defined('WPLANG')?'1':'0').':'.$locale.':'.apply_filters('locale',$locale)."\r\n",3,'debuglog.txt');
	//endmod

	if (empty($locale))
		$locale = 'en_US';

	$locale = apply_filters('locale', $locale);

	return $locale;
}

and in http://mu.wordpress.org/forums/topic.php?id=2042&page=2#post-67728 i have said:
"i have added "?l=en" like links to multilingual home theme. current language is saved in cookie."

but it is hard to modify multilingual home themes, so i want to change single language home themes.

but proper way is http://codex.wordpress.org/Plugin_API i think.

"&&!headers_sent()" is not in correct place in this code. may be it causes wrong language on some pages. it should be there:
if(!headers_sent())setcookie(...);
and it works without them at all.

this code sets multiple cookies.
fix:
global $locale, $flagkukiisset;

if($_GET['l']=='l1'){
			$locale='l1_CO';
			if($flagkukiisset==0)setcookie('forum_guest_language','Lang1',time()+365*24*60*60,'/','domain.tld');
			$flagkukiisset=1;
		}elseif($_GET['l']=='l2'){
			$locale='l2_CO';
			if($flagkukiisset==0)setcookie('forum_guest_language','Lang2',time()+365*24*60*60,'/','domain.tld');
			$flagkukiisset=1;
		}else{
...

new version of that file to download, with this fix and changes in readme file, but theme files are still 2.65 version: http://tmf.org.ru/download/file.php?id=30 . md5:019a16fdd944ee36dcf0d10835425388, 7z file size:54609bytes.