The PLC doesn't use UTF-16 internally.
It is only the i-TRiLOGI software that is using the UTF-16 so that comments and menu can be displayed in international languages. The PLC doesn't really care what you use. So if you need to display something in UTF-16 you need to send two bytes to form a single Unicode characters and if your hardware support it then you can display it.
E.g. Some Chinese LCDs are able to accept data sent in to it as Unicode and display the characters accordingly and the PLC can then send the required codes and i-TRiLOGI does provide the translation utility for Chinese characters under the "Help" menu. For other languages there are online translator that let you input the international language characters and translate into the two-byte numbers that represent the character and you can then use them inside your program. Unfortunately you would not be able to view these characters using online monitoring.