You say Dracula talks like an ‘American teenager’, but it is established in the first episode that when Dracula drinks somebody’s drug, he ‘downloads’ (if you will) knowledge of their culture, including the ability to fluently speak their language. Presumably, then, he learned to speak fluent teen by feeding on Lucy Westenra.
Yes I get the logic behind the change but it did nothing for the character, in fact it took away his appeal that was built over the 2 episodes, he was no longer menacing or seemingly interested in causing chaos and havoc, he just seemed so pedestrian and that ending and explanations why things happened was unsatisfying for me