I would like to improve my English as much as possible during the short time that I have before I go back to my country.
ご質問ありがとうございます。
"I would like to improve my English"＝「私は自分の英語を上手くしたいです」
"as much as possible"＝「出来る限り」
"during the short time that I have before I go back to my country."=「帰国するまでの短い間に。」
(☆"short time"の代わりに"short period"でもOKです）
ご参考になると幸いです！
アメリカ合衆国 
