UTF-8 encoding has been introduced into XOOPS 2.3 as default charset. However, there might be some problems with converting existing websites from non UTF-8 charset to UTF-8.
Before there is a good enough solution for this conversion, following settings are recommended when you upgrade an existing website if you are not an experienced user:
Select "Do not change" option in "Database character set and collation" step during upgrade process
Modify /languages/yourlanguage/global.php to use your previous _CHARSET value, if it has been changed to UTF-8 in your new global.php file as define('_CHARSET', 'UTF-8');