There are three parts to adding a filetype:
1. making a custom filetype file defining a language optionally using an existing Scintilla lexer for highlighting (not sure if any will work, or just get confused by Julia syntax) and an existing uctags parser for symbol recognition (though I doubt any will actually recognise Juila types `Array{Quantity{Int64,D,U} where U where D,1}` or do the level of inferencing Julia does)
2. add a Lexer to Scintilla and add it to Geany and move the filetype to internal.
3. add a symbol parser to uctags and integrate it into Geany
Pull requests are welcome (AFAIK no existing contributors use Julia, so somebody needs to contribute it).