What should I use? UTF8 or UTF16?

I have to distribute my app internationally.

Let's say I have a control (like a memo) where the user enters some text. The user can be Japanese, Russian, Canadian, etc. I want to save the string to disk as TXT file for later use. I will use MY OWN function to write the text and not something like TMemo.SaveToFile().

How do I want to save the string to disk? In UTF8 or UTF16 format?

Thanks