Agree that alphabetic is the only sensible _default_ menu division. To illustrate categories, look at the Wikipedia categorical section, and look at how many categories each language appears in. It is meaningless. The Geany team should be humble enough to not be programming language wizards and enforce some categorisation, just use alphabetic.
If somebody is soooo convinced that they need non-alphabetical they can make a separate PR built on this that reads a conf file to replace alphabetic and they can do whatever they want.