Q:

Seeking to work and relocate to Japan; would knowing Japanese improve my chances?

I just graduated and got hired in April in Germany as software engineer.
However, I have wanted since my university days to study/work in Japan, in a video games firm, and this personal ambition is becoming obsessional.
I do not possess experience in the games industry, neither have I lived in Japan.
I am willing to work on games ( personal projects ). It is already hard enough to get a job in this industry in Germany with my background. 
Would a better knowledge in Japanese change anything to my marketability?

Clarifications based on comments:

What exactly are you looking for, and what concerns do you have, and what have you done so far about it?

I am looking for work, and I know I have technical & language-related short-comings. I only have passed  JLPT n5 last year. That's worrying me.

What about American or English companies?

Thank you but I'm not interested in working in Europe/NA. 
I feel attracted to the local culture. Some people like to move around, to live some kind of thrill.
Until now, I have tried to get introduced in the Japanese market through the help of professors or various programs (Vulcanus for example) but to no avail. 
I am trying to assess my chances to find a job by myself. 
I would like to thank DarkCygnus for this great answer and ShinEmperor for this technical answer. I will read those books ! 

I would not expect a good salary, or good working conditions in the video game industry in Japan. I would only recommend this path for you if you really want to make Japanese games, and are willing to put up with a low quality of life (in terms of income and work/life balance) in order to do so.

This is an important remark. I will think of it. 

A:

In general, in Japan there are companies that operate in Japanese, and companies that operate in English.
Unforturnately for your situation, the vast majorities of companies in Japan operate in Japanese, so if you could not communicate in Japanese, it would vastly limit your choices. That said, even companies that operate in Japanese may make an exception for someone with exceptional techical skills if they have someone who can communicate in English within their organization. (This is an exception, not common practice)
For those that operate in English, Japanese is often not a requirement. However, even then, being able to speak/read/write basic Japanese would be recommended as companies would give preference to those with at least a general knowledge of Japanese.
So if you really have your heart set on working in the video game industry in Japan, I would say you have two options.

Learn Japanese in order to increase the number of positions you would qualify for.
Become very skilled in game development, and make a name for yourself in the field in order to increase your likelyhood of being hired at the limited number of positions available to people who do not know Japanese.

While not directly related to the question, I would like to point out that the percentage of people who can speak English proficiently in Japan is relatively small, so if you plan on living in Japan for any extended period of time, I think you would get more out if it if you knew the langauge.

UPDATE:
Based on your update, I see that you already have JLPT5. While that wouldn't hold much meaning in a business world, it does show that you are making an active attempt to learn the language in your home country, which would earn respect from many interviewers. Learn how to do a basic self-introduction, and be sure to get your hiragana/katakana down and at least a few kanji. Be polite and respectful in your language, and you may be able to find a smaller company who wants developers to pick you up and sponsor a visa for you. 
Saying this, I would consider working at a company other than a game company as your first company in Japan. Getting your foot in the door is the hardest part, and limiting yourself to game companies that would hire a non-Japanese speaker as your first visa-sponsor would prove difficult, I think. If you are willing to spend a year or so at a Japanese startup, or small development company, I think that it would be easier to transfer to a game company showing that you have experience working in a Japanese company, and you would already have a visa, so it would be easier and faster for them to be able to hire you. Of course they would still have to renew your visa when the time comes, but there is a higher chance of approval for a renewal than the first visa application, assuming there were no issues with the law or tax evasion.

A:

Would a better knowledge in Japanese change anything to my marketability ?

If you are aiming to get a job at Japan, knowing Japanese will most definitely be of aid for you and your chances of landing the job.
Nowadays, most people can speak and understand English, and most likely in the target company you seek they do. But, speaking Japanese (the official language of the country) will surely give you a great advantage over candidates that don't.
Besides that, knowing Japanese will also let you interact and live your every day life more smoothly, and be able to adapt faster to the relocation, as opposed to not knowing Japanese.
Complementary to that, given that you have experience in software development, I would try to hone my skills, knowledge, and tools that are usually involved in game development (say, some Graphics Framework, state-of-the-art techniques, etc.). This, along knowing Japanese, will increase your chances of landing the job significantly. 

A:

No matter what you level of Japanese is, better knowledge of Japanese would without a doubt improve your marketability.
Learning the local language will help you in any business in any country in any time period and always will. The question isn't "Will it help you?" but "How much will it help you?" and "What level of Japanese is absolutely necessary?". Its utility can range anywhere from extremely useful to absolutely necessary.
Germany and Japan are opposite ends of the spectrum (among developed countries) when it comes to English proficiency. In Germany, practically every educated person speaks fluent English. German technical business is conducted in English. This is not the case in Japan—not even in Tokyo. Most Japanese people are not comfortable speaking English and are not fluent in it. Japanese firms conduct internal affairs in Japanese.
It is possible to work in Japan at a Japanese company without knowing Japanese for one of two reasons. Either (1) you lead a foreign business with a lot of money and will pay a premium for them to do the translating or (2) you have valuable skills and the company you are working for is willing to pay pay the translational overhead of employing you. Neither of these is likely to be the case for a videogame company because lots of software engineers want to work on videogames. Videogame companies can therefore take their pick of software engineers in a way most other tech industries cannot.
To summarize:

Japanese companies perform internal business in Japanese.
Videogame companies do not face a severe labor shortage.
Therefore Japanese videogame companies are more likely to hire an engineer who speaks Japanese over one who does not.

You should still learn Japanese even if you never attain a high proficiency. The reason is interpersonal. Imagine a Japanese person coming to Berlin and expecting you to speak him/her in Japanese. That'd be unreasonable, right? Well, that's basically what you're doing if you go to Japan and expect the local people to work with you in English. (It's not quite as bad since English is the world's international language but I hope I'm getting the right idea across here.) Japan has a high standard for politeness. Attempting to speak Japanese is the polite thing to do. You should try as hard as you can to speak Japanese. Japanese people know Japanese is hard. They'll give you big points for trying because so few foreigners even try.
For more information, I recommend Inside the Kaisha: Demystifying Japanese Business Behavior by Noboru Yoshimura and Philip Anderson.
Disclaimer: I have neither lived nor worked in Japan. However, my family has Japanese roots, I've traveled to Japan, I read + speak Chinese, I've lived in China and I can read more Japanese than most expats. I have studied Japanese culture and history.

