My eldest son is 14 years old and has been living in Germany ever since we moved there five years ago.
When he first arrived, he did not speak a word of German. Now, he can speak fluent German, Vietnamese and English, as well as conversational Russian. However, I have noticed that he was beginning to have some trouble forming more complicated Vietnamese sentences, and he cannot spell very well in his mother tongue.
I try to let him speak at least some conversational Vietnamese at home. I teach him Vietnamese every day, and explain as best I can when he makes mistakes. Nevertheless, my son’s default language is now German. He responds in German during sudden outbursts of emotions. To him, German feels more natural than Vietnamese, which is not surprising, as he uses the language every day at school. Sometimes, he shies away from responding to me in Vietnamese because he does not want to make mistakes and face my language lessons.
Admittedly, Vietnamese has its own quirks. We have different words to describe "uncle" and "aunt", and all sorts of different interpersonal relationships. If people do not grow up in Vietnam, pronouns can be very confusing.
My younger child speaks a mixture of Vietnamese and German, as his Vietnamese vocabulary is not enough to fully express what he wants to say. He talks to himself in German, not Vietnamese.
Of course, I do not want my children to forget Vietnamese. I make calls to our relatives in Vietnam almost every other day, and invite my children in for brief conversations so that they can become more acquainted with their Vietnamese family. I also take my children home to Vietnam every summer. I spend time with our family in Vietnam, and take my children to visit monuments in Ho Chi Minh City. I hope they always remember where they were born.
My firstborn stills love every bit of Vietnam that we have in our daily life. He loves Vietnamese food, which he loves sharing with his friends in Germany. He also loves traditional dance from the Mekong Delta and is now practicing a routine to perform at school. My younger child is more used to Western food, which is understandable as he grew up here. However, I am not too worried, I am sure he will enjoy Vietnamese stuff like his brother once he grows up.
All and all, I believe that despite a life in the West, young Vietnamese living abroad generally will not lose their roots. It is okay to let them adapt with local culture first before teaching them about other traditional values from their homeland.