## 4 Discussion
### Language  
The review shows that English, particularly US English, is frequently used as the language of choice in child-speech-synthesis systems. However, English is often used due to the market size [3] and the availability of English data required for training. Good-quality recordings, in addition to the phonetic and linguistic knowledge that is required for the annotated text resources in the language, can come at a high cost and unfortunately, they do not exist for some languages [70]. Some researchers have attempted to overcome these difficulties in adult-speech synthesis by crowdsourcing speech samples [71], using available data such as audiobook data [16] and by using nonideal corpora. In some cases, where a bootstrapping technique can be used, data from a major language can be shared with data from a new language if the new language is comparable with a major language. In those cases, researchers can create synthetic voices for low-resource languages with a small amount of training data [72]. For example, since Mandarin and Tibetan belong to the Sino-Tibetan language family, the speech data and speech-synthesis framework can be borrowed from Mandarin, whilst only making use of a small amount of Tibetan training data [72]. This could potentially be a useful technique for the building of synthetic voices in the Bantu languages of South Africa. In a recent study, results were encouraging when working with data from children speaking in a second language or a non-native language, i.e., Italian children speaking both English and German and German students speaking English [73]. A multi-lingual data adaptation in transfer learning and multi-task learning framework was found to be useful [73]. Nevertheless, there are an overwhelming number of languages that have yet to be considered for speech synthesis.  
As English is often prioritised for speech synthesis, children who do not speak English, along with those who do not use it as a first language, are often disadvantaged. This is particularly apparent in South Africa because if a child has to use a pre-loaded voice through an AAC device, English could very well be the child's second or third language [7]. As children with CCN may also have comorbid language difficulties, the limited language options subsequently place an additional barrier on their communication. It is well established that an individual's voice is unique and can signify particular elements, such as their physical size, age, sex, race, intellectual ability, geographical and social background, as well as their personality [4; 64; 74]. In many countries, including South Africa, a person's language also plays a role in their identity by representing their culture. Thus, the effect that a language selected for an SGD could have on a child's identity should not be minimised. It is also important to remember that for children with CCN who need to make use of SGDs, speech synthesis not only forms the basis of their personal identity, it is also crucial for communication and an essential component of social interaction [3]. If the language on their device is different from those in their immediate social environment, then the child's communication effectiveness is negatively influenced.