A little advice

Your friend is correct...no pause from college, finish that first, then study the language by devoting all the time to it. You cannot over learn in acquiring the language, the more the better.

Finish college first.

Could you continue learning Japanese while finishing your engineering degree at home ? Maybe do some extra reading in your specialization in Japanese journals ? Rather than asking random (but well-meaning) strangers, could you discuss this with an academic advisor -- perhaps a professor who would be writing your recommendation for graduate school?

Finish your studies first otherwise you will not be fully focused upon your return.

Finish, it is tough to go back after a year or two off. Plus your college may have some restrictions especially in engineering. Use the extra year to study and find a Japanese social group to assist with your language.

I agree about finishing school first. But If I was going to be an engineer, I would be placing emphasis on getting my P.E. credentials before I did anything else. That would make you more marketable And make more money.
If I was going to spend a lot of money and a year of my life learning a foreign language, it would be the secondary language where I would want to spend my career and live. If you never speak the language, it won't stay with you.

Getting the PE doesn't work because it requires 5 years (I believe) of experience prior to the exam. You can always pick up the PE later if needed. And another question is how are you financing this? I assume you have that covered. And, as has been suggested, you need to be sure of your college's graduation requirements. When my engineering son applied for a study abroad in his junior year he encountered an old rule of his engineering school that required a residency on campus for the last four semester of his program. It took a fair amount of effort to get that rule waved so that he could spend a semester in Spain. You program may not require that but double and triple checking is always a good idea.

Taking a year off to travel and study a foreign language before you start college or after one or two years is not a horrible idea. Taking a year off when close to finishing never seems to work out well. Since you're close, finish college first.

Not sure where you're studying, but I do know that in some engineering schools it is next to impossible to get into classes you want if you get "off cycle" in the schedule. So taking a break could add more than just one year to your schooling. If you do take a break, double and triple check with your college about the impact before taking the leap, and get any agreements in writing if possible.

Thank you all for your answers, after reading, staying seems not to be as bad idea as I first thought, still I'm gonna double check with my school.
thanks again and have a nice day :D

Finish the college first. You have time to study more Japanese until December next year, at least you could get your way around in the beginning with the language. Next year you'll be 22, a bright future ahead :D

Here is something that might interest you. The Japanese government operates a program, known as JET, which places native English speakers as assistants in secondary schools. The remuneration varies by prefecture, but I can tell you that my daughter received a salary and lodgings. You would have plenty of time for your own language classes, and nothing beats immersion for developing language skills. Check with your college placement office or on google.