Learning multiple languages at once?

So, /lang/, I seem to have a proclivity to languages. Whenever I read something concerning a language, I just remember it if it's interesting, regardless of how foreign it was. I haven't touched Dutch in over a week and half, and I simple said a few words a few times and I remember them perfectly. I just remember things. I was wondering if it was feasible to learn one or two languages on the side in addition to my class-based language that I'm taking. I understand that it can be quite an obstacle to learn two languages at once, but I'm in no hurry to learn them, so I'm not particularly worried about losing a great amount of speed on progress. I want to eventually learn Japanese, assuming that *chans will always be a part of my life until either they or I go, assuming the latter will occur first, it would/will be useful to know Japanese, also considering Japan has the second highest GDP, around 1/3 plus a little more of America's.

So /lang/, would learning multiple languages at once ruin the entire process, or would it just make them slow?

Learning two languages with similar grammar and vocabulary would make things somewhat easier. I wouldn't recommend it but since time is no concern to you and you really have an interest in learning more than one at once, go for it.

Also, Japan's GDP may be the second highest but it won't lead the region in the 21st century and it isn't the 80's anymore. If you asked me which Asian language would be more beneficial in finding a job, I'd say Mandarin Chinese would be it.