This text is in Unicode, why is it showing up as question marks or missing character symbols?

There are two reasons why this could be happening.

The first is that while Unicode is a character set that tells your computer which characters to display your computer still needs a font to determine what these characters actually look like. Most modern computers come with all the Unicode fonts for the most widely used languages, but there are some that need their own font to be added. 

Fonts for some of the lesser used scripts in Unicode can be found here.

The other reason this text might be garbled is that it is not truly Unicode. When a modern browser receives non-Unicode text when it is expecting Unicode it will try its best to turn that text into Unicode, but depending on the original character set of the text this might not be possible. What results is a text that uses the Unicode character set that because it was translated incorrectly no longer holds any meaning.