The lecture will be arranged when you come to Japan between next May and April.
We will arrange the discourse during your visit to Japan between next April to May.
質問ありがとうございます。
「講演は来年の4月から5月の日本滞在中にアレンジします」は
The talk will be arranged during your visit to Japan between next April to May. でも通じますよ!
または、
❶ The lecture will be arranged when you come to Japan between next May and April.
(講演は来年の4月から5月あなたが日本に来るときにアレンジします)。
❷ We will arrange the discourse during your visit to Japan between next April to May.
(講演は来年の4月から5月のあなたの日本滞在中に私たちでアレンジします)。
とも言えますよ、参考に!