Most tech companies in Japan use Japanese. But that doesn’t mean opportunities for non-Japanese speakers don’t exist. I created this job board listing such opportunities.
Still, if you are interested in Japan, I’d suggest studying some basic Japanese, as showing you’ve made the effort will help separate you from other people. As a first goal, I’d target being able to introduce yourself in Japanese. That level is probably going to be the best return on investment in time studying, and is a realistic goal in months not years.
As a new graduate it is going to be a bit of a challenge to get a job here. Japanese companies generally don’t hire developers from overseas, and when they do, they tend to be looking for ones who already have professional experience. I did recently advertise a company that’s doing an internship for recent grads. This kind of opportunity is exceptional, but if you were to land it it would be a good first step to a job in Japan.
One thing I noticed is that the website linked from your GitHub profile is down. I’d either resurrect it or remove the link. Maybe you were already doing it on that site, but I’d recommend building up a public presence. That you’ve done GSoC is great, but not everyone is going to know the significance of it. So you might write about an article about your experience doing it, from how you were accepted to it (I think there is an application process) to what you accomplished.