Could you learn a different language by reading a book in the language?
I'm trying to learn both German and Japanese at the same time, and there's this series of visual novels that I like that come out in English and Japanese (the creators of the visual novels are Japanese). Would it be worth while to get one or more of the Japanese apps of theirs? Right now I have the English version and Japanese version of a couple of their apps. Could that be helpful or am I wasting my time? I'm also wondering because if this could be helpful, it would be awesome to find books in German that I like, like the German version of Harry Potter or something.
If you're good at comparing sentences cross-linguistically, I think it would be a worthwhile investment because you can learn a lot in that way.
You can also use this site, which offers classic literature like Alice in Wonderland in multiple languages and you can read two translations side-by-side. http://bilinguis.com/
You can learn a lot that way, but a word of caution: Do not read something that is older than 1950 or is specifically targeted at children. Even then, some old children's books are not suitable. Someone here recommended "Alice in Wonderland" and this is a book I only recommend when you can already fluently read newspapers in the target language. In other words: When you have already learned that language and try to further improve your skills.
I always recommend the Asterix comics. They are easy stories, funny, and are translated into more languages than any other comic I know, including dead langauges (like Latin), several dying languages and even dialects (editions in various german dialects are available).
Other than that, I recommend easy reading books targeted at young people. My foreign language breakthrough to English were Dragonlance novels. When it comes to Japanese, things are difficult. There are very, very few bilingual books available and at least two of them are too tough for learners of the language.
Reading is excellent for learning a language, but you have to know the basics (unless it's a language very similar to your mother tongue). Since you're learning German, I can recommend you these series of easy to read books especially designed for beginners: "Dino Lernt Deutsch" They helped me a lot. You can find them on iBooks and other similar places. Here's the first chapter: https://youtu.be/vUXcYTjINtI
Books are a great way to learn (reading comprehension of) a language. However, I don't advise you to compare two versions of a book while you're reading. Just read the book in English, and then read it again in Japanese/German. I used this technique myself when I was studying Spanish, read Harry Potter in Spanish, it was great because I learned a lot of vocabulary about magics and stuff.
There is just one little thing that might be difficult. You have to have a more or less good grammar and vocabulary background if you want to do so. You can't for instance read Harry Potter if you don't know half the words and the structures used in it. Also, when doing so, try not to systematically translate what you're reading to English.