Hello. I'm Japanese. I answer your question. Hopefully means うまくいけば "umakuikeba" in Japanese. So I tanslate this sentence like this, 暑くなったら、うまくいけば、（私は）泳げるでしょう。"atsukunattara, umakuikeba, (watashiha) oyogeru deshou".

Hopefully I'll be able to swim when the weather gets hotter. As Xingtian-san wrote, hopefully would be translated to うまくいけば. Also, we often say like this: 暑くなって、泳げるといいなあ。 It literally means that that'll be good if I'll be able to swim when the weather gets hotter. Hopefully the weather will be fine. 天気（てんき）がよくなると、いいなあ（と思（おも）います）。 Hopefully we'll enjoy our trip to Japan. 日本への旅（たび）が楽（たの）しいものになると、いいなあ（と思います）。 I think this is one of colloquial expressions and is used often. I hope this will help you too!