Your sentence would be translated as: هذه اللغة جميلة.
The logic is a bit different between the one they put and the one you typed.
In their example, notice that IS comes after THIS. In English, the verb (to be) which is "IS" in this case, separates between the subject and the predicative (in other words, between what we want to talk about, and what we are trying to say about it).
So, in their sentence, the subject is considered "THIS" ... and "pretty" is an attributive adjective to the word "language" (an adjective the describes the language and part of it).
In your sentence, the subject is "this language" altogether. The adjective "pretty" is alone, because it is the predicative of the sentence (or the item of information we are telling).