You’ve chosen languages with a high degree of difficulty (except for the English language).
Let’s start talking about the Arabic language. When I was learning French I met an Arabic pen pal on a language site, she was an Arabic teacher. She told me that the Arabic language is quite complex and possesses an extreme degree of difficult even for native speakers. What I’m trying to say is that the Arabic language is a long term learning process.
Read more here:http://www1.folha.uol.com.br/ilustrissi ... rabe.shtml
Japanese and Chinese:
These two are languages that require long term learning process. You need around of 2,200 hours of learning for each of them. The same amount of hours you will need to learn Arabic.
Japanese:http://noticias.universia.com.br/destaq ... studo.html
German (medium degree of difficulty):
Now we are talking about a language with a lesser degree of difficulty if we compare it to the three languages I’ve mentioned above. You will need something around 750 hours.
Here there’s a language difficulty ranking:http://www.effectivelanguagelearning.co ... difficulty
I think the easiest one for you to achieve your goal is the English language if you study on a daily basis.
Thus summarizing my opinion: it’s not impossible. But it will take an incredible amount of time, devotion and patience. If you organize a schedule e.g. English (two years already) study four hours a week and German ( 10 hours a week) during two years. After that you decide to learn Japanese. You keep studying English 4 hours a week, German 8 hours a week and Japanese 12 hours a week and so on. You can’t stop studying the first language because you should keep learning and developing your vocabulary, listening, reading and speaking.
In my opinion it’s quite a challenge. It reminds me the beauty and complexity of a classic music “Night on bald mountain” by Modest Mussorgsky (Russian composer).
P.S. I’ve written in English because I personally think you should read in English.