When you input text to TextField with Japanese IME the textChanged() signal is emitted only when 'enter' key is pressed and not when Japanese characters are input. The 'enter' key is used to accept Japanese characters e.g. from Kanji list, but it is not required to input Japanese.
For reference, textChanged() signal is emitted with every (Latin and Japanese) character in TextArea.
The attached example contains a TextField (with "Type here" placeholder text) and a TextArea for testing. Debug message is printed in onTextChanged slot. See the difference between TextField and TextArea with Japanese IME.
Tested only on Windows 7 and 8.