Though the Dramatica software is in English (including the help materials), you can easily WRITE in Spanish (and many other languages) without any problems. In fact, we have many international users writing in many different languages: Spanish, French, Dutch, Italian, Swedish, German, etc. I suggest downloading the demo software and try it out before buying it.
You may also be interested in reading a book in progress by one of our Mexico City-based Dramatica users. He is a native Spanish speaker and writes in English and Spanish. The book he is writing is in English, but you may find it interesting: Dramatica for Screenwriters.
NOTE: It's important to point out that the Dramatica software does not work properly with most multi-byte languages, such as Japanese, Chinese, Hebrew, Arabic, among others. Perhaps this limitation will be lifted in some future version of the software.