When I lived in Japan and visited Korea I sometimes spoke Japanese by accident, because my brain defaults to it when in a place where I don’t know the language. Have done the same in Thailand. It’s either not understood or if it is, kind of offensive/amusing.
Later I moved to Korea to teach English and have been told I speak Korean with a Japanese accent, which is 90% amusing, 10% kind of offensive to people. It’s hard to switch my brain to read hangul the Korean way rather than the hiragana way. My Korean accent is woeful. Like I was telling the taxi driver I wanted to go to Makudonarudosu the Japanese way of saying McDonalds, and he was like, WTF? Eventually, oooh… Maegdonaldeu. Me: why can’t I speak to people here
Though they speak waaaaay better English in Korea than in Japan. The first time I went, I got out of the airport and seeking noms and unsure of what foods existed there or how to buy them, went to the most familiar place I could see, Mr Donut. I walked in and the cashier said, “Hi, how can I help you?” in English, and I just gaped at her and didn’t know how to respond. No one had spoken English to me at a counter in like two years at that point. So I am trying to stop my brain from replying in Japanese but also didn’t really know what to say so I just sort of froze up while she looked at me like I was some kind of crazy person. It took me by surprise. In Seoul, lots of people will just speak to you in English.
To answer your question, it’s not really useful, no. If you visit China it will help you read signs on doors though!