From OMIM
Speech-language disorder-1 is an autosomal dominant disorder characterized by severe orofacial dyspraxia resulting in largely incomprehensible speech. Affected individuals were originally thought to have specific defects in the use of grammatical suffixation rules (Gopnik, 1990; Gopnik and Crago, 1991). The phenotype, however, is broader in nature, with virtually every aspect of grammar and language affected (Fisher et al., 1998). Vargha-Khadem et al. (1998) concluded that the disorder is characterized by abnormal development of several brain areas critical for both orofacial movements and sequential articulation, resulting in marked disruption of speech and expressive language. Relation to Specific Language Impairment Children who fail to develop expressive and/or receptive language normally, in the absence of explanatory factors such as neurologic disorders, hearing impairment, or lack of adequate opportunity, are clinically described as having specific language impairment (SLI; see 606711) (Bartlett et al., 2002). See also familial developmental dysphasia (600117).  http://www.omim.org/entry/602081

From MedlinePlus Genetics
FOXP2-related speech and language disorder affects the development of speech and language beginning in early childhood. Affected individuals have a condition known as childhood apraxia of speech (CAS), which makes it difficult to produce the sequences of sounds and syllables needed to form words. CAS is caused by abnormalities in the parts of the brain that plan and coordinate the movements of the lips, mouth, and tongue. Children with FOXP2-related speech and language disorder say their first words later than other children, typically between 18 months and 7 years of age. Their speech is often difficult to understand, although the clarity of speech usually improves over time.  

In addition to having problems with producing speech (expressive language), people with FOXP2-related speech and language disorder may have difficulty understanding speech (receptive language). Some affected individuals also have trouble with other language-related skills, such as reading, spelling, and grammar. 

Less commonly, individuals with FOXP2-related speech and language disorder have features of autism spectrum disorder, which is a condition characterized by impaired social skills and communication problems. Some affected individuals have difficulty with motor skills such as walking, writing, or buttoning clothes, but these typically improve with treatment. Some affected individuals may have learning difficulties.   https://medlineplus.gov/genetics/condition/foxp2-related-speech-and-language-disorder