What language should I speak in dealing with the Japanese
Most people who wanted to learn Japanese in dealing with the Japanese. However, most Japanese who speak English they want to associate with the English speaking people.

Which is the better language to deal with the Japanese ? My nephew he married a Japanese girl, but her mother denied her own identity, claiming that she don't speak Japanese at all. I noticed that her mother always talk Japanese with her own family members. My question is: why Japanese considered themself non-Japanese and speaking Japanese is a shame?

Question

What language should I speak in dealing with the Japanese
Most people who wanted to learn Japanese in dealing with the Japanese. However, most Japanese who speak English they want to associate with the English speaking people. 

Which is the better language to deal with the Japanese ?  My nephew he married a Japanese girl, but her mother denied her own identity, claiming that she don't speak Japanese at all. I noticed that her mother always talk Japanese with her own family members. My question is: why Japanese considered themself non-Japanese and speaking Japanese is a shame?

Answer

Not all Japanese are what you think.
You shouldn't decide things when you see only a part.

Answer

Hi... Try to speak the language the person wants to hear ... that's all.

Answer

eeeeh ...
Japanese consider themselves Japanese because it's their nationality.  
and Speaking Japanese is not a shame because it's primary language.

In your case , your nephew can only speak English and his Japanese wife must have spoken English to communicate with him.  I take it for granted to talk with the language which the partner can best use for communication.  To get along is a kind of implicit virtues.

and English is good enough to communicate . Actually I have 1 friend who worked for AEON (one of the biggest English conversation school)
but she could survive without Japanese language though she lived in extraordinary countryside. That means there are , at least , a  few volunteers and sociable people who can speak English even countryside because it's one of our subjects in compulsory education.

Answer

Whichever you speak more fluently and accurately.

As you may know, Japanese language is highly context-depending language. We put very subtle nuance and emotion carefully between lines.  Therefore, if you want to do business with Japanese people in Japanese language, you have to be really fluent almost in native level. Otherwise there would happen miscommunication, misunderstanding, and loss of business. Business is different from friendship.

Nevertheless, we feel very happy when foreigners somewhat speaks Japanese or even just interested in Japan. For this reason, learning Japanese would be a great help to make good relationship.

Answer

I probably did not understand your question correctly but... of course I would try to speak japanese if I can speak the language. Even if your japanese is not very good... 
If you are not able to get your point across you could always try english. Not everyone might understand english .. but I would say that your chances are pretty good.

Edit: 
I think alucky got the answer. If I speak with someone I usally pick the language that I think they understand best. Or the language that most of the people understand in a group. So maybe her mother did not want to make it difficult for you. Or she wanted to practise another language.

Ne manquez pas cette occasion d'apprendre une langue depuis chez vous. Découvrez notre sélection de professeurs de langues expérimentés et inscrivez-vous dès maintenant à votre premier cours !