Anthropological linguists generally accept the principle that all languages and dialects are equally effective as systems of communication. What is the term used to describe this principle?

Principle of linguistic relativity is the correct answer.

Generally, anthropological linguists accept the idea that all languages are equally effective as systems of communication and you cannot rank them as being better or worse. In addition, the principle of linguistic relativity, also known as Sapir-Whorf hypothesis, suggests that the structure of a language (or dialect) affects worldwide speakers' world views or cognition. So, people's perceptions will be relative to the language they speak.