If you were running monkey
at some point, it probably changed the input method — it happens quite often.
You can change the input method by long-pressing on an input field and choosing Input Method → Android Keyboard. I think you can also long-press on the bottom-left key on the virtual keyboard.
Note you can also disable the Japanese and other input methods from being activated entirely via Settings → Keyboard → untick the box next to each IME.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…