καθυστέρηση λόγου → speech retardation

[Vasilis]

διαγνωστική εκτίμηση παιδιών με καθυστέρηση λόγου

καθυστέρηση λόγου → speech retardation;

[billberg23]

I suspect this is correct, referring to children who are slow in learning to speak (cf. http://pediatrics.aappublications.org/content/16/2/272.abstract).  Ordinarily, however, slowed-down verbal expression (through stuttering, malformation of vocal organs, etc.) is referred to as speech impediment (cf. http://www.thefreedictionary.com/speech+impediments).

[mavrodon]

Και speech/oral impairments/impediments

[Vasilis]

speech impediment or speech impairment/disorder is probably «διαταραχή λόγου».

[mavrodon]

Retarded speech/vocal retardation/verbal retardation.
Επίσης: speech/vocal/verbal hesitency.