Dear Joanna,
Landau-Kleffner Syndrome is re;atively rare, and is characterized by both aphasia and an abnormal EEG (though some children with LKS do not display clinical sizures). The aphasia can occur suddenly or in a more gradual manner.
Thus, if you daughter's EEG's have been entirely normal, she does not meet one of the criteria for LKS. The condition can be difficult to diagnose, and can easily be regarded as a problem with auditory processing or some other form of learning disability. One of the significant aspects of a correct diagnosis is the implication re: medication. A child who only displays a problem with auditory processing, for example, is not a candidate for medication. A child who truly displays LKS, on the other hand, is likely to be prescribed medication. LKS is a neurological disorder.
I don't know what her diagnostic workup will show, relative to how specific it can be.
A child your daughter's age has a normal developmental thrust to expand their language and communication skills, so it can be quite frusrating when they encounter obstacles. A key requirement for you as a parent is to maintain your own equanimity and patience; this will help your daughter. Perhaps some of our readers who have a child who displays LKS will weigh in and mention how they cope with the day-to-day interactions with their children.