I didn't know a single word of Italian before going. That was a big mistake on my part because it took so much longer to connect with my host family and the other Italians in my city. However, because I went with School Year Abroad (
www.sya.org) there were 60 other American high school students or post graduates to provide a support network and the school really worked hard to make sure we learned Italian. From what I hear from Rotary members, the immersion is much more intense because there are few other international students with you. However, Rotary does provide very good support.
The bottom line is if you really feel that you're independent enough, work hard, laugh often, and are willing to spend a few months lost, there will come a time (usually in December) where the language just clicks. The transition period is tough, really tough, but then that's why Rotary gives you a whole school year and part of what makes studying abroad so special! I still remember that moment I realized I had stopped translating in my head and started really THINKING in Italian. Now that was amazing.
I went with no expectations of how things should run, so I had no problem adapting to a new style of life and culture.
As far as college goes, I agree that ASAP does bring up a good point. Applying to colleges from Italy was a hassle. I needed to email things back and forth to my parents, my teachers, my guidance counselors. I also had to explain on all my applications why my Secondary School Report was coming from my home school in Southern California but my Midyear School Report was coming from a school in Italy. The support from abroad was not great either because let's face it, the American college admissions system is vastly different from any other country. My host family, for example, couldn't understand why it was such a stressful process or why extracurricular activities were so important. But, it's doable. Gather all your materials before you leave, write a couple of essays so you can get the feel for it and have it looked over by your guidance counselor, and CONTACT COLLEGES BY PHONE AND EMAIL.
I learned a lot in Italy. And even only three weeks in I was able to tap into all my emotions and experiences to write a great college essay that I'm sure helped set me apart form the thousands of other college bound grads. If you choose to go to another country senior year, explain yourself clearly, and really learn, grow, mature, I think you're college chances wouldn't be hurt at all.
Good luck!