Re: Does Tikiwiki support UTF-8?
Since I can't convince you, I hope you can test this youself by install Joomla and Tikiwiki in the same server to see the fact.
If you use Windows OS, it is very easy to build a testing server with XAMPP, which offers you PHP 5.2x and MySQL 5.0x. (I also used MySQL 5.0x in my test).
It is good that you speak Japanese. Please just input some Japanese texts into Joomla and Tikiwiki respectively, then you export the SQL file and check the difference. You can even install Joomla 1.5.14 and Tikiwiki 3.1 in the same database to see the bug of Tikiwiki.
Please do it and you will understand what I had said.