As a Japanese faculty at a reputed school I get a lot of people asking me the same question - "What is the fastest way to learn Japanese?". You might be one of those guys.
You might just need to learn Japanese because you are about to go to a Japanese speaking nation, or because you are looking for marketing in the Japanese speaking market, or may be just because there is a new girl in your class who is more comfortable with Japanese.
I am fluent in Spanish too, so I can tell you that mastering any language takes time. However, if you can get to follow a right guided approach, you can learn to work around with any language in a matter of few weeks. However such stuff can be quite costly.
But, for all those of you who are on budget but want to learn Japanese in less than eight weeks, I recommend Rocket Japanese. With a laser guided approach this course can take you post intermediate level in less than eight weeks. You can read the detail review below.
For those of you who are on zero budget, I suggest you to start learning Japanese with help on this site. We have covered every topic on learning Japanese and its available to you for free.
If you still have any queries or need more help, just mail to me at: nick@learnJapanesereviews.net.
Good Luck, Friend!