I tried this during my weeb phase some 20 years ago.

I stumbled across a video lecture series om some torrent site, and despite being very old (fr m the 70s or 80s) it was actually pretty good for teaching everyday japanese.

I never progressed beyond the very basics due to life happening, but it got me far enough that I could at least grasp the general topic at hand. I’m sure I would’ve gotten a decent understanding of the language if I had kept at it.

Japanese is a fairly simple language with easy grammar. From what little Mandarin I’ve learned, I’d say the two are far enough apart that knowing one probably won’t help you much with the other.