Serna uses unmodified GNU Aspell spell checker that is included under the GNU LGPL license (see ASPELL-COPYING in the installation directory). This implies that license terms are applicable to the spelling library aspell-*.*, located in the plugins/speller directory of the installation and to the files residing in plugins/speller/data files.
You can download sources for the library either from its official web site, or from the Syntext web site.
The Serna package includes the English word-list for Aspell, maintained by Kevin Atkinson, see copyright details in ASPELL-EN-COPYING in the plugins/speller/dict directory.