You can define a "FontSelector" and then add your fonts (fontSelector.AddFont). Now iText will choose the not available char from the next defined font by using the fontSelector.Process method.
Asian characters are usually Unicode so you have to set "BaseFont.IDENTITY_H" instead of Ansi ones (such as FontFactory.GetFont("tahoma", BaseFont.IDENTITY_H)).
If it's RTL, you need to wrap your text in elements which have run direction property and set it to rtl.