Psychological behavior of talking alone in front of mirror. My child, 5 years old girl recently talked to herself about she just changed to new school in front of mirror. It was alone and she just talked to herself on mirror. Is it psychologically normal condition or what should parents do about it?
By itself, a single incident like this, or even a habit of talking to oneself in the mirror, doesn't qualify as a mental disorder, in the opinion of one psychiatrist.
For a bit more background on the research in this area, there's a NYT article. Alas it doesn't get into children... and children to have more behavior that few adults have, like imaginary friends.
So the contents of the external self-talk is probably more important than the mere fact she does it.
The introductory part of an article focused on self-talk in Down syndrome mentions that

Self-talk or talking out loud to oneself (also referred to as 'private speech') is a common behavior in young children and plays an important role in the development of higher-level thinking and self-directional skills. Developmentally, children engage in self-talk to coordinate their actions and thoughts. During focused goal-directed activities, self-talk/private speech systematically becomes more selective in 4-year-old children than in 3 year olds[1]. By the age of 6 or 7 years, most children stop talking out loud to themselves and develop an internal dialogue when working out problems or challenging tasks[2].
In certain situations, people engage in self-talk (either positive or negative) during periods of stress or conflict or to motivate themselves. Negative self-talk has been found to increase during levels of affective stress in children and adolescents with anxiety and depression[3,4]. Studies of the use of self-talk in people engaged in exercise and competitive sports have found that exercisers and athletes use self-talk as a positive, motivating behavior to enhance their performance[5,6,7]. Few studies have been conducted on self-talk behavior in people with intellectual disabilities. Self-talk has been a common observation by parents of children and adults with Down syndrome[8,9]. It was postulated that self-talk is an important aspect of the thought processes of people with Down syndrome when they are working out difficult situations, expressing feelings, and entertaining themselves[10]. No association was found between self-talk/private speech and behavior problems, communication difficulties, or social isolation. However, self-talk in adults with Down syndrome has sometimes been incorrectly considered to be hallucinatory behavior [11,12].