When using it for Devoxx Belgium 2018, I get:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at org.optaplanner.examples.conferencescheduling.persistence.ConferenceSchedulingCfpDevoxxImporter.importTalkList(ConferenceSchedulingCfpDevoxxImporter.java:316)
	at org.optaplanner.examples.conferencescheduling.persistence.ConferenceSchedulingCfpDevoxxImporter.importSolution(ConferenceSchedulingCfpDevoxxImporter.java:95)

That should just work for a random user.

Overwriting it to use a local talks.json file should happen through a (namespaced) system property, which you can set in the IntellJ run configuration in the VM options field as for example -Dorg.optaplanner.examples.conferencescheduling.importFromCFP.talkOverrideFile=local/talks.json". That file should just be created with new File(...) so it starts from the working directory if it doesn't start with /.
Do we want to have multiple system properties, one per json file or just one to signal the json files directory? (as you see fit)

Additionally, make an inventory of reasons why the devoxx-cfp importer is too devoxx Belgium specific and won't just work for Devoxx France.