Characters such as á é ü are considered to be non-ASCII characters and will not display correctly in text fields if included in an XML file. If you are coding your own XML file and intend to include non-ASCII characters, you will need to encode them as character entities. You can reference any character entity chart for these numerical entity values, or use the Postable utility to convert them for you.
For example, to include a caption with an á character, your XML file should appear similar to this:
<img src="image.jpg" caption="á" />