If you could decide a topic for our next week's English conversation, we could prepare what to say in advance.
I think that would be good for the lesson.
ご質問ありがとうございます。
"If you could decide a topic for our next week's English conversation,"＝「来週の英会話のためにテーマをあなたが決めてくれたら」
"we could prepare what to say in advance."＝「事前に何を言うか準備できます。」
"I think that would be good for the lesson."＝「それはレッスンに良いことだと思います。」
ご参考に。
アメリカ合衆国 
