See the manual and the HACKING file.
Highlighting is provided by the Scintilla editing widget Geany uses which does not support Prolog, so it would need to be added there (written in C++), see https://www.scintilla.org/Lexilla.html.
For symbol parsing Geany uses the parsers from the Universal Ctags project which also does not support Prolog. There was an attempt to add it but it appears to have been abandoned, see https://github.com/universal-ctags/ctags/issues/2628