Flatten FDF / XFDF forms to PDF in PHP with utf-8 characters

Question

My scenario:

• A PDF template with formfields: template.pdf
• An XFDF file that contains the data to be filled in: fieldData.xfdf

Now I need to have these to files combined & flattened. pdftk does the job easily within php:

exec("pdftk template.pdf fill_form fieldData.xfdf output flatFile.pdf flatten");

Unfortunately this does not work with full utf-8 support. For example: Cyrillic and greek letters get scrambled. I used Arial for this, with an unicode character set.

How can I accomplish to flatten my unicode files? Is there any other tool that offers unicode support? Does pdftk have an unicode switch that I am missing?