How to store international characters in XML?

Question

I've a problem whereby the XML that I've generated contains:

<attribute name="Actors">Trevante Rhodes , André Holland , Janelle Monáe</attribute>

but it should be this:

<attribute name="Actors">Trevante Rhodes , André Holland , Janelle Monáe</attribute>

@IntelliData's answer to the question posted here seems to suggest that the only solution is to replace them with their HTML encoded counter parts.

So... if I have a source file that may contain any foreign character from one or more countries, does this mean that I'd have to write a routine to do just that?

And if I have no choice, is there a more efficient way to do this search and replace?

myData = myData.Replace("À","À").Replace("à","à").Replace(...)

MORE INFO

To create the XML file, I am using

using (StreamWriter sw = new StreamWriter(myTxtFile))
{
...
}

myTxtFile thus contains

 Trevante Rhodes , André Holland , Janelle Monáe , Ashton...

I then open the .txt file using:

using (StreamReader sr = new StreamReader(mySourceFile, Encoding.GetEncoding("iso-8859-1")))
{
...
some processing to assign the data to XML nodes...
...
}

and finally create the XML file:

using (StreamWriter sw = new StreamWriter(xmlDataFilename, false, Encoding.GetEncoding("utf-8"))) 
{
...
myXMLObject.outerXml
...
}

which produces this:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<productRequest>
    <attribute name="Actors"> Trevante Rhodes , André Holland , Janelle Monáe , Ashton...</attribute>
</productRequest>

MORE INFO 2

The answer to the search and replace question lies here