Learning a foreign language/culture will impair the learning of one’s mother tongue and culture?

I think it depends on what kind of language. For example: If your regular language is english, learning languages like German, French or Spanish may appeal easily, but different languages like Chinese or Arabic will be hard. If Chinese is your best language, you may find Japanese fairly easy, but English fairly hard. It all depends on what your mother tongue is.

学习一门外语/外国文化会损害母语和文化的学习么？

No, I don't think so. I think in some sense, learning the foreign language/culture can enhance one's understanding of mother tongue and culture, since languages/cultures have many similar contents and meanings.

I think it is extremely terrible!