In general, TV can help your child enhance their language if caregivers:
make sure the child has the basic language skills needed to understand the language presented on TV,
direct TV viewing to programs that are language-rich, and
co-view the program with the child to create opportunities for asking/answering questions (e.g., "what do you think will happen next?"), introducing new words ("that's a skyscraper, it's a really tall building"), and building connections (e.g., "that reminds me of when we went camping").
First, make sure that the language content of the program is at an appropriate level for your child. Children need to have the language before the TV show can reinforce it or expand it. Children learn by listening and encouraged active participation. Children need to hear words a lot before they will start using them. This is why programs that are predictable, repetitive and the vocabulary is at the 12 - 18 month old level (e.g., teletubbies) are enjoyed by toddlers. If you would like your preschool child to enjoy and learn from shows with higher language skills (e.g., "Franklin" or "Arthur"), it is best to read them stories with these characters first and sit down and watch a few episodes with your child talking about the program while it is on and after it has finished.