Language declaration question

["Hewko, Doug" <Doug.Hewko@ccra-adrc.gc.ca>]

Out of curiousity, when you specify an encoding value in a XML document,
where does the XML processor obtain the encoding values from? Would the
processor return an error, automatically download the encoding value, or
does it come already with every known encoding value? (I have difficulty
believing the latter.)

For example:

1) I am programming XML for your MS IE 5.5 browser (assume it supports the
ISO-8859-6 values). I happen to specify the an Arabic language coding,
ISO-8859-6. What would someone get if they have the basic American version
of Windows? 

2) This time I am programming server-side, in Cocoon (or Saxon, whatever).
Same type of question. The server supports the languages (don't know how but
let's say it does). What would a user get when accessing a XML document in a
different language?