I spent three years teaching English (two in Sapporo as an ALT and one in Nagoya as an Emergency Teacher or ET). I was initially offered the ET position during the recruitment process in Chicago because I had some experience with the language from my first two years in Sapporo. As an ALT I had lots of free time which allowed me to study/socialize so the language came pretty quickly. Typically, ETs are promoted from within the ranks of AEON branch school teachers who had been there for 2-3 years. I was the exception in this case due to my language ability which translated to a higher base salary, daily per diem, etc.
During my time with AEON I travelled to over 17 different schools while on assignment and I noticed that every branch school seemed to take on the personality of the manager. Some schools absolutely forbade foreign staff from speaking Japanese during working hours even between staff with no students present. At the same time, foreign teachers are expected to give the other Japanese teachers at their school a regular English lesson every week as part of their duties (a bit hypocritical if you ask me).
Other schools didn't mind the fact that I used Japanese as long as it was restricted to the lobby or in private conversations. And some schools (few and far between though) actually encouraged me to use Japanese as much as possible (God bless the great people at Yokkaichi! 8-p) in front of students, between staff, etc. It all boiled down to the manager and her (I say "her" because all 17 managers were women) philosophy on what was "appropriate", heh.
Bottom line is had I not put forth the effort to learn as much Japanese as possible on my own, I doubt I would have been able to go into business for myself after having spent only 3.5 years in Japan. My school officially opened 6 weeks ago and I already have 19 students, heh. I also advertise in all of my promotions that I often use Japanese in class when the need arises. Judging from the enormous positive feedback i've had from both students and prospectives alike, I would say i've been pretty fortunate.