In fact, it can be argued that the work of Christian missions has actually helped preserve some aspects of indigenous cultures. This is perhaps most obvious in the case of language, as an important part of missionary work is the translation of the Christian scriptures into local languages.
According to Kate Shellnut, the author of an article that recently appeared in Christianity Today (see here), about a third of the more than 7,000 languages that exist in the world are endangered--with a language being lost every 40 days. This is at least in part due to the fact the speakers of these endangered languages are forced to use the dominant language in their country to engage in commerce and to take advantage of social services like education and health care, as is noted in the article by Gary Simons of the Christian linguistic organization SIL. However, the work of translating the Bible into the languages of the people being evangelized often has facilitated the preservation of endangered languages, by creating written records of those languages. Moreover, the creation of a Bible translation in a particular language can lend that language a certain prestige, which can encourage people to continue to use that language.
Indeed, it is clear that from the Bible itself that the language of each unique people group is of value in the eyes of God. In Revelation, the last book of the Bible, the author John writes of a "great multitude that no one could count, from every nation, tribe, people, and language" (Rev. 7:9, NIV) worshipping God in Heaven. Moreover, efforts to translate the Scriptures into languages other than the original Hebrew and Greek have existed almost since the beginning of Christianity. There is no sense that a Bible translated into, say, French or Chinese, is any less the Word of God than one in its original languages (at least since the Reformation!). Contrast this with Islam, in which a Quran is only considered to be the true revelation of Allah if it is in Arabic.
Image of American missionaries to Micronesia, 1852, from Wikimedia Commons