Your first challenge is that you state you're "very bad at finding jobs". Finding a job in Japan is going to be more challenging than in France, as there are going to be even less opportunities available to you. However, you also identify how to get a job ("randomly through contacts"). You have a far better chance at getting a job through someone you know than firing off your resume to some random place on the internet.
The first thing I'd consider is if there is any way you could work remotely for a company in France. I'd talk to all your past colleagues, and ask them if they know of any opportunities. You could also do freelancing projects.
If you are looking for a job in Japan, being Hiroshima is a blessing and a curse. While there are far less opportunities than Tokyo, there are also less people searching for developer jobs (especially foreigners). I'd use that to your advantage. Identify interesting looking companies in Hiroshima, and contact them.
Ideally you want to contact someone working there directly (Twitter can be a great way of contacting Japanese developers). Rather than trying to get hired at their company, mention you're a developer who's recently moved to Hiroshima, and are wanting to get to know the local developer community, so could you take them out for lunch or a beer. By connecting directly to people, you have a far higher chance in getting a response.
Similarly, I'd try going to some local events to make connections. Hiroshima MotionControl Network and Web Touch Meeting are two I found - attend them and you can ask about others.
If you do fire your resume off to a random company, and you don't write a really compelling application, chances are they'll ignore you. To do this, you need to write a custom application for each company you apply to. Plan to spend at least a couple of hours on it. Research the company, and identify what kinds of things are important to them. Based on that, write a tailored application emphasising how you match them.
I'd also not give up just because you didn't hear back from a company. Follow up with them on a regular basis until you do hear something back. It's easy for an application to fall through the cracks, and there is nothing wrong with being persistent.
One thing that did pop out at me was you said you "rarely use 3rd parties". For companies like mine who heavily use open source, this can be a negative. I'm looking to hire developers who would rather use a third party library than reinvent their own thing, as I think from a long term cost perspective, it is far cheaper to use something written by someone else. Of course, you also need to be good a judging if this library is good or not, as you're right that sometimes having an extra dependency can introduce more complexity. My point is, if you put that you preferred to write stuff yourself in an application to me, I'd be less likely to hire you. You could find out my views by looking at my GitHub profile for instance, which while not incredibly active does include some open source contributions.
More generally, I think a great resource on the topic of getting a job as a developer in adverse situations is Hired Fast. It elaborates on some techniques for standing out. Even if you don't buy the book, you can sign up for the mailing list and get some tips on getting hired.