Well, you can ask Scintilla for the enhancement (or better yet supply a patch) to support the non-standard `_"` syntax of Scheme, or use the standard syntax from Lisp as [shown](https://www.gnu.org/software/gettext/manual/html_node/Common-Lisp.html#Commo...) which Scheme gettext also supports according to your link above.
Agree about SF.net, not because I don't like it, but because it screwed up a password change and refuses to reset or recover, so I can't use it. And another Geany contributor has the same problem. But you could submit your patch to Scintilla by email to its mailing list.