Is it possible for a person to lose the use of their native language? Studies in linguistics (which I did a bit of in my college days) say yes, under some conditions. Language is more than a means to engage in communication and commerce: it defines us more than anything else, and acts as our "operating system," to put it in computing terms. After the Islamic Revolution in Iran in 1979, many Iranian students studying in the U.S. at the time were forced to stay permanently. With little hope of returning to their home country, some accepted their new life so completely that they stopped using their native language, until they essentially became unable to speak Farsi properly. During my time in Japan, I've never felt I was losing my ability to speak English, although I did go for more than a year without talking to a native speaker, which meant the English I was speaking was being subtly affected by my ESL students, which was quite a weird experience. It's amazing how hard it can be to recall certain seldom-used words when you live in a non-English speaking country like Japan, such as "potentiometer" or "gynecological," and at times I've been obsessed with trying to recall a word I knew existed but which I couldn't remember for some reason.
Naturally, it's not uncommon for Japan to affect the people who choose to live here in strange ways.