Essentially none, I would think. There might be a rare exception where anxiety or severe perseverative behaviors are a barrier to language development and Zoloft would treat that specific symptom, but I don't know anyone that would start Zoloft with the specific goal of accelerating language development.

Thanks for your answer Bradley. A parent was invited to enrol her child in a study in the US and asked me. I didn't find any specific studies either. 

Narrative Structure and Narrative

Development in Deaf Children

CHRISTIAN RATHMANN, Centre for Deaf Studies, University of Bristol,

UK; WOLFGANG MANN, Deafness Cognition and Language Research

Centre, UCL, UK; GARY MORGAN, Language and Communication

Science, City University London, UK  2007

Narrativas infantis em Língua Brasileira de Sinais. 

Maia Cristina da Cunha Pereira y Ricardo Nakasato. Letras de hoje. Porto Alegre. V.39. N°3. pp. 1-378. 2004

Thanks for the resources Dora.