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base languages are the ones that exist in your xoops/languages directory, such as english, french
for Chinese, it could be schinese(simplified chinese), tchinese(traditional chinese) or uchinese(UTF-8 Chinese)
extended languages are the ones that converted from corresponding base language.
For example, I have english and schinese available with my XOOPS hence the base languages are: english, schinese
I would provide simplified chinese, traditional chinese and utf-8 chinese for visitors, so I set extended languages: tchinese, utf8chinese, and the corresponding base language must be schinese from which the extended language characters will be converted.