Hi, welcome to the Inkscape forum!
What kind of input method are you using? On my Linux system I use
UIM, which should work on a Mac as well. For Japanse input I use the Anthy package, but I don't know whether there is an equivalent package for Macs.
Note hat Inkscape has some strange behaviour regarding this input method — before choosing the eventual type (i.e. hiragana, katakana or kanji), the text appears
in the upper left corner:

After confirming, it moves to the right place:
