Also it might be worth surveying what other editors/IDEs do to see if there's any other good approaches.
A limited survey of those I could find online docs for is either alphabetical menu (atom, eclipse) or type it yourself (vim and emacs) or no such command (or their online docs are #$%^ so I didn't find it) (sublime, qt creator, anjuta)
Another more obvious way would be to group them alphabetically, collapsing ranges of letters together where there aren't very many languages (to avoid a large 26 item menu with few items in each submenu).
Yeah, and the advantage of alphabetical is that nobody can complain that their favourite language is disadvantaged by calling it a "script" language not a "programming language", or calling something like Julia a Pascal language, yes yes I know Julia isn't in Geany yet, but its an example of an up and coming language with a different syntax style, there will always be new styles invented.