I don't think you'll come back with a Japanese accent...but you will start speaking English with Japanese patterns. My mom came to visit for my last three weeks in Japan, and we took the time to do a lot of traveling, which meant I had to serve as translator. She's got a bad habit of trying to hold a conversation with you while you're engaged in something else--it's a big aggravation when I'm on the phone--but it was especially difficult here because I'd be speaking Japanese and listening to her English while waiting for a reply in Japanese from the person I was trying to hold a conversation with. If it was something I'd have to translate for her, a lot of the times I'd have to try three times to get it out in English that she could understand...I'd reverse adjectives and nouns, start a sentence with a direct object, stuff like that. As an added bonus I became less expressive in English because my vocabulary started to drop due to my not hearing it much. I was in Japan for about the same length of time, so I imagine this will happen to you, too.
The best part of the whole thing was that it was only my English that broke down...never my Japanese (at least outside of mistakes I was working to correct). It's an interesting feeling when the skill level in your native language begins to drop....
