Problems with special chars when extracting Mail

I am using this script for saving a mail out from the Apple Mail app:

tell application "Mail"
    
    # take the selected Mail
    set selectedMessages to selected messages of message viewer 0
    set mailToSave to first item of selectedMessages
    
    set resultFile to (choose file name with prompt "Speichere E-Mail unter ..." default name "Mail.eml") as rich text
    if resultFile does not end with ".eml" then set resultFile to resultFile & ".eml"
    
    my writeTextToFile(source of mailToSave, resultFile, true)
    
end tell

on writeTextToFile(theText, theFile, overwriteExistingContent)
    try
        
        -- Convert the file to a string
        set theFile to theFile as string
        
        -- Open the file for writing
        set theOpenedFile to open for access file theFile with write permission
        
        -- Clear the file if content should be overwritten
        if overwriteExistingContent is true then set eof of theOpenedFile to 0
        
        -- Write the new content to the file
        write theText to theOpenedFile starting at eof
        
        -- Close the file
        close access theOpenedFile
        
        -- Return a boolean indicating that writing was successful
        return true
        
        -- Handle a write error
    on error
        
        -- Close the file
        try
            close access file theFile
        on error errMsg
            log errMsg
        end try
        
        -- Return a boolean indicating that writing failed
        return false
    end try
end writeTextToFile

In most of the cases it's working perfectly, but it some cases the special chars are broken in the exported file. For example:

One mail is displayed correctly in Mail. If I open the source of this mail I'll get this:

MIME-Version: 1.0
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="UTF-8"
X-Mailer: .......

...

für Sie ...

In Mail the text is correctly shown as a für Sie ....

The exported file contains:

MIME-Version: 1.0
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="UTF-8"
X-Mailer: ...

...

fÌ1Ú4r Sie ...

If I drag and drop this mail out of Apple Mail, it's getting saved correctly. Any way how I can achieve this with AppleScript?

I figured out an own way as I am now using JavaScript:

var source = mailToSave.source();
var newSource = decodeURIComponent(escape(source));

To get the final result, I need to save it as an UTF-8-String, what is only working with NSString:

writeTextToFile(newFilePath.toString(), newSource);

function writeTextToFile(pathString, textToWrite) { 
    // convert to NSString
    var nsString = $.NSString.alloc.initWithUTF8String(textToWrite)
  
    // convert to NSPath
    var nsPath = $(pathString).stringByStandardizingPath
  
    // write File
    var success = nsString.writeToFileAtomicallyEncodingError(nsPath, false, $.NSUTF8StringEncoding, null)
  
    if (!success) {
        throw new Error("Write failed. ERROR: Writing to file failed.\nPath: " + pathString)
    }
  
    return success
}