include_once XOOPS_ROOT_PATH.'/modules/xlanguage/api.php';

into XOOPS/include/common.php
RIGHT BEFORE
// #################### Include site-wide lang file ##################
if ( file_exists(XOOPS_ROOT_PATH."/language/".$xoopsConfig['language']."/global.php") ) {
include_once XOOPS_ROOT_PATH."/language/".$xoopsConfig['language']."/global.php";
} else {
include_once XOOPS_ROOT_PATH."/language/english/global.php";
}

base 1: name: english; description(optional): English; charset: iso-8859-1; code: en (or anyother like "xen", not a true language code, just the tag for indicating English content)

base 2: name: schinese; description(optional): Simplified Chinese; charset: gb2312; code: zh (or anyother like "sc", not a true language code, just the tag for indicating Chinese content)

extended lang of schinese 1: name: tchinese; description(optional): Traditional Chinese; charset: big5 code: zh-TW (the true language code of Traditional Chinese)

extended lang of schinese 2: name: utf8; description(optional): Simplified Chinese UTF-8; charset: utf-8 code: zh-CN (the true language code of Simplified Chinese)

wrap content of each language with respective tag specified in step 3:

[langcode1]Content of the language1[/langcode1] [langcode2]Content of the language2[/langcode2] [langcode3]Content of the language3[/langcode3] ...

if two or more languages have same content, you do not need add them one by one but use delimiter "|":
[langcode1|langcode2]Content shared by language1&2[/langcode1|langcode2] [langcode3]Content of the language3[/langcode3] ...

a true example (suppose the lang_codes specified in step 3 are: English-en; French-fr; SimplifiedChiense-sc):

[en]My XOOPS[/en][fr]Moi XOOPS[/fr][sc]我的XOOPS[/sc]

OR:
[en|fr]This is my content in English and French[/en|fr][sc]中文内容[/sc]