I do not understand the reason either, but give an example in Japanese Joomla!.

1. All Joomla! files are stored in UTF-8 no BOM.

2. Setting of PHP is as follows.

default_charset UTF-8
mbstring.language neutral
mbstring.internal_encoding UTF-8
mbstring.encoding_translation Off
mbstring.http_input auto
mbstring.http_output pass

3. All databases are UTF-8.　

But there may be the case that a problem happens if you make it these settings.

Please send PHP setting of your mbstring and iconv.

Thanks.