I've found out what was causing trouble with bad database encoding using MySQL database. All posts with national characters (???���...) contained question marks instead of national characters. It's because original files storing DB config like mysql.properties and jforum-custom.conf have bad connection strings for the UTF-8 encoding.
The whole string for UTF-8 encoding is lowercase by default but it HAS TO BE UPPERCASE like "UTF-8" in dbencoding property.
Of course you have to use mysql specific connection settings like mysql.useUnicode and mysql.encoding because MySQL needs to know the encoding of stored data. Since then all problems dissapeared and all post now show correct characters.
Bellow are the important functional setting from configuration files: