All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Islam allows a man to marry as many as four wives provided he feels capable of being just between them. Allaah Says (what means): {…then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one….}[Quran 4:3]
Therefore, if you are able to be just, you may marry a second wife; it even becomes compulsory if you fear to commit adultery. If this Japanese woman is Muslim, religious and of noble character, or she is a chaste Christian or Jew, then try to convince your mother to allow you to marry her. Tell her that the criteria for marriage in Islam are the religion of a woman and her character not her nationality or anything else. Inform your mother that this marriage may yield beneficial results.
If your mother becomes satisfied with you and accepts your second marriage, then praise be to Allaah, otherwise you have to give priority to the obedience of your mother over your marriage to this woman, because obeying your mother in lawful matters is compulsory while marrying a specific woman is not compulsory. Hence, what is compulsory takes precedence over what is not so. There are many other women; so search for another one, and Allaah may bless you with a woman who is better than this one. Allaah Says (what means): {But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allaah Knows, while you know not.}[Quran 2:216]
Allaah Knows best.