I think that when a child is adopted from another country, they can keep their culture from their home country, but I think it depends on the adoptive parents. The adoptive parents can encourage and teach the adopted child's culture if they want. Sometimes they might not and that would make it that much harder for the child to learn their culture.
A child and their family have every right to try to maintain as much of their culture as possible. It is good to practice their home language and customs. With that being said, it is very difficult to do when moving to another country. Although some traditions can be maintained, it is also essential to learn to adapt to your current environment. It is impossible to do either 100 percent of the time.