Learning a new language is not a bad thing, but it really doesn't work for incels. I learned Japanese because I already knew how to read/write Chinese so Japanese was among one of the most low-hanging fruits. I use it mostly to read raw hentai RPG (which is kind of important especially when there's puzzle-solving) and occasionally LN/VNs (though I have to go very slowly).
On the other hand, I would never know if Japanese actually increases my SMV since I don't really talk to foids and they don't really talk to me. I have no way to advertise my language skills. I remember going to Japan and asking the locals for directions in Japanese, they just dismissively pointed their fingers at the direction and moved on. The longest conversation I've had with a Japanese was a foid who tried scamming us with a claw machine.
I've seen on TV where a white chad says arigatou or nihao with the accent of a caveman and still have foids drooling over him. Don't expect the same to happen to you.