I’m from Vietnam. I’ve been in the UK for 10 years now. When I met my English husband 13 years ago at 19 I knew 0 English. We communicated using machine translation. So that’s when I started learning English. Fast forward to present day after immersion, living in an English speaking country, formal study, etc. and I’d say my writing and listening (understanding) are good, but my speaking and reading are still bad. I kind of gave up on trying to become fluent at this point.
Have you tried learning Japanese / English after learning the other? I studied Japanese and learned how to pronounce the /r/ in Japanese correctly.
For some people, the difficulty is less in production, and more in interpretation for someone who is native Japanese speaking and later learned English.