Windows XP: How can I get the system language from command-line?

This is done by REG QUERY HKLM\System\CurrentControlSet\Control\Nls\Language /v InstallLanguage

Then you get a number with the following meanings (no guarantee of completeness and correctness):

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" 

If you want to actually test for the system language, you might consider using WMIC:

wmic.exe os get locale, oslanguage, codeset

yields on my machine:

CodeSet  Locale  OSLanguage
1252     0409    1033

For instance, if you wanted to test for Japanese (locale 0411), you might place the following in a batch program:

wmic os  get locale, oslanguage,  codeset | grep 0411 > nul && @echo Japanese Language installed!

Test for both locale's and issue goto's if they are found. Error out if they are not.

You're probably looking for a Locale rather than the OSLanguage or Codeset, but i included them here because of the way your question and the accepted answer are worded.