the words意即某人说的话.如Please tell me in your own words.
这里,words是复数形式,所以其谓语或系词须用复数.
they use：这里是一个定语从句,用来修饰the words.
谓语的单复数形式与宾语无关,而只跟主语相关.如：
You are a boy.
所以,本句为：
the words they use are called a language.