"Diversity awareness training" is presumably something that a "seasoned account manager" has already experienced possibly has even helped to present to employees working under him. It is an important if not essential part of the training that should be provided to all persons involved with public contact that may include individuals with different backgrounds (which is all of us!).
The scenario you are presenting, however, isn't concerned with a lack of recognition of the diversity of background. It is addressing a difficulty in communication due to differences in pronunciation or language patterns between the employee and the customers. This problem needs to be addressed through provision of further training and practice in use of the English language with emphasis on the vocabulary most likely to be used by this employee in his/her contact with the public.