Importing Data Containing Special Characters
XML Files Can Have Any Character Encoding
XML files can be saved with any character encoding. That character encoding should be defined at the top of the XML file, per XML standards on specifying the character encoding of an XML document.
If special characters in your XML file aren’t appearing correctly in the posts created by WP All Import, it means that the encoding specified in your XML file is probably incorrect – maybe your XML file is claiming to be a UTF-8 when it really is a Windows-1252, or similar.
CSV Files Must Be UTF-8 Or UTF-8 Compatible
Since there is no way to specify the character encoding for a CSV file, WP All Import just assumes all uploaded CSV files are UTF-8, which is the standard character encoding for the web.
This works the vast majority of the time, but if you are importing a CSV file with special characters (things like umlauts, non-Latin characters, etc.) and those special characters aren’t appearing correctly in the posts created by WP All Import, try opening your CSV file in a text editor using the correct encoding, and then re-save it using UTF-8.
On a Mac, our favorite way to do this is using Sublime Text 2.