I wrote to Baggio in the usual airmail envelope sold outside. Generally in English, I believe everyone can understand it. Write the address of the other party in the lower right corner of the envelope, and write the return address in the upper left corner or back. If you ask someone for an autograph, the contents of the letter should be written in German in a friendly way. You go to the post office and ask people how much it costs to send a letter to Germany.
Baggio wrote back to me three months later, when he was still in Inter Milan. Generally, it takes more than 10 days for a letter to get there from here. So be patient!