easy english

"The effect" {subject} and "this" {noun} can be used together due to the grammatical acceptance of this type of sentence structure.
Another example will be "The feeling this song brings out is peaceful." In which "The feeling" is the subject and "this song" is the noun.
For "The English spoken in USA is different from that spoken in England." Using "which" instead if "that" is not possible,"that" is a replacement for "the English" and cannot be replaced with "which" {a prerogative pronoun}.
This sentence actually means:
The English spoken in the US is different from the English spoken in England.