In line 9 and 10 the essaist wants to support his prevoius statement (English is the richest for the purposes of writing poetry) explaining that he doesn't refer to the quantity of great poetry England produced, clarifing his thought.
Then he repeats his thesis in a easier way and it seems that he is starting to provide the reasons why he agrees with the topic sentence saying English has the largest vocabulary but in the next line the essaist denies his preovious statements saying it is only a consequence of the real reason that makes the English the best language. In line 16 the writer provides the real reason that supports his thesis: the variety of elements of which English is made up. So from line 17 to line 30 the essaist starts to explain and justify his preovious sentence listing all the elements of which English is formed.