Dictionary.Load is immediate. It's not technically "loading" from a stream. It's taking the already stored JSON string in memory and then converting the JSON into the Dictionary storing mechanism. So there is no need for when Dictionary is finished as it's finished the moment the action is done. you can start using data in the same event.
Start -> AJAX.Load( file )
AJAX.OnLoad -> Dictionary.Load( Ajax.lastdata )
-> Dictionary is ready to use.