つまり、何が言いたいかというとって英語でなんて言うの?
何かを伝えるとき、色々説明して、相手が、話の要点がわからなくなっていそうなときに、言う言葉です。
回答
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In short, what I'd like to say is (that) ...............
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In other words, what I want to say is (that) ...........
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In short, what I want to tell you is that ...........
"つまり、何が言いたいかというと"
つまり in short, in other words, in brief
言いたいことは what I'd like to say is ...., what I want to say is ....
伝えたいことは what I'd like to tell you is ..., what I want to tell you is ....
In short, what I'd like to say is (that) ...............
In other words, what I want to say is (that) ...........
In short, what I want to tell you is that ...........