Languages and Windows Registry Codes

To change the whole system language you should install newer service pack with different language.
For example if you have XP SP2 in ENG and want to have it in Italian, you will have to download SP3 in Italian, change some registry keys (go to HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Nls\Language registry key, and then change the value date for “Default” and “InstallLanguage” registry value from 0409 to 0410) and you'll be able to install it in Italian.
If no problems occur, you will have the whole system language changed. Here is the list of numbers that represents each language in registry: I

0436 = "af;Afrikaans"
041C = "sq;Albanian"
0001 = "ar;Arabic"
0401 = "ar-sa;Arabic (Saudi Arabia)"
0801 = "ar-iq;Arabic (Iraq)"
0C01 = "ar-eg;Arabic (Egypt)"
1001 = "ar-ly;Arabic (Libya)"
1401 = "ar-dz;Arabic (Algeria)"
1801 = "ar-ma;Arabic (Morocco)"
1C01 = "ar-tn;Arabic (Tunisia)"
2001 = "ar-om;Arabic (Oman)"
2401 = "ar-ye;Arabic (Yemen)"
2801 = "ar-sy;Arabic (Syria)"
2C01 = "ar-jo;Arabic (Jordan)"
3001 = "ar-lb;Arabic (Lebanon)"
3401 = "ar-kw;Arabic (Kuwait)"
3801 = "ar-ae;Arabic (you.A.E.)"
3C01 = "ar-bh;Arabic (Bahrain)"
4001 = "ar-qa;Arabic (Qatar)"
042D = "eu;Basque"
0402 = "bg;Bulgarian"
0423 = "be;Belarusian"
0403 = "ca;Catalan"
0004 = "zh;Chinese"
0404 = "zh-tw;Chinese (Taiwan)"
0804 = "zh-cn;Chinese (China)"
0C04 = "zh-hk;Chinese (Hong Kong SAR)"
1004 = "zh-sg;Chinese (Singapore)"
041A = "hr;Croatian"
0405 = "cs;Czech"
0406 = "the;Danish"
0413 = "nl;Dutch (Netherlands)"
0813 = "nl-be;Dutch (Belgium)"
0009 = "en;English"
0409 = "en-us;English (United States)"
0809 = "en-gb;English (United Kingdom)"
0C09 = "en-au;English (Australia)"
1009 = "en-ca;English (Canada)"
1409 = "en-nz;English (New Zealand)"
1809 = "en-ie;English (Ireland)"
1C09 = "en-za;English (South Africa)"
2009 = "en-jm;English (Jamaica)"
2809 = "en-bz;English (Belize)"
2C09 = "en-tt;English (Trinidad)"
0425 = "et;Estonian"
0438 = "fo;Faeroese"
0429 = "fa;Farsi"
040B = "fi;Finnish"
040C = "fr;French (France)"
080C = "fr-be;French (Belgium)"
0C0C = "fr-ca;French (Canada)"
100C = "fr-ch;French (Switzerland)"
140C = "fr-lu;French (Luxembourg)"
043C = "gd;Gaelic"
0407 = "de;German (Germany)"
0807 = "de-ch;German (Switzerland)"
0C07 = "de-at;German (Austria)"
1007 = "de-lu;German (Luxembourg)"
1407 = "de-li;German (Liechtenstein)"
0408 = "el;Greek"
040D = "he;Hebrew"
0439 = "hi;Hindi"
040E = "hu;Hungarian"
040F = "is;Icelandic"
0421 = "in;Indonesian"
0410 = "it;Italian (Italy)"
0810 = "it-ch;Italian (Switzerland)"
0411 = "ja;Japanese"
0412 = "ko;Korean"
0426 = "lv;Latvian"
0427 = "lt;Lithuanian"
042F = "mk;FYRO Macedonian"
043E = "ms;Malay (Malaysia)"
043A = "mt;Maltese" 0414 = "no;Norwegian (Bokmal)"
0814 = "no;Norwegian (Nynorsk)"
0415 = "pl;Polish"
0416 = "pt-br;Portuguese (Brazil)"
0816 = "pt;Portuguese (Portugal)"
0417 = "rm;Rhaeto-Romanic"
0418 = "ro;Romanian"
0818 = "ro-mo;Romanian (Moldova)"
0419 = "ru;Russian"
0819 = "ru-mo;Russian (Moldova)"
0C1A = "sr;Serbian (Cyrillic)"
081A = "sr;Serbian (Latin)"
041B = "sk;Slovak"
0424 = "sl;Slovenian"
042E = "sb;Sorbian"
040A = "es;Spanish (Traditional Sort)"
080A = "es-mx;Spanish (Mexico)"
0C0A = "es;Spanish (International Sort)"
100A = "es-gt;Spanish (Guatemala)"
140A = "es-cr;Spanish (Costa Rica)"
180A = "es-pa;Spanish (Panama)"
1C0A = "es-do;Spanish (Dominican Republic)"
200A = "es-ve;Spanish (Venezuela)"
240A = "es-co;Spanish (Colombia)"
280A = "es-pe;Spanish (Peru)"
2C0A = "es-ar;Spanish (Argentina)"
300A = "es-ec;Spanish (Ecuador)"
340A = "es-cl;Spanish (Chile)"
380A = "es-uy;Spanish (Uruguay)"
3C0A = "es-py;Spanish (Paraguay)"
400A = "es-bo;Spanish (Bolivia)"
440A = "es-sv;Spanish (El Salvador)"
480A = "es-hn;Spanish (Honduras)"
4C0A = "es-ni;Spanish (Nicaragua)"
500A = "es-pr;Spanish (Puerto Rico)"
0430 = "sx;Sutu"
041D = "sv;Swedish"
081D = "sv-fi;Swedish (Finland)"
041E = "th;Thai"
0431 = "ts;Tsonga"
0432 = "tn;Tswana"
041F = "tr;Turkish"
0422 = "uk;Ukrainian"
0420 = "your;Urdu"
042A = "vi;Vietnamese"
0434 = "xh;Xhosa"
043D = "ji;Yiddish"
0435 = "zu;Zulu"