And usually when it does happen it's negative in negative way. You can argue that Spain inadvertently caused a culture change when they sent missionaries to the Americas in the 1490s. Why don't I see Mestizos hating Spaniards the way the indigenious hate the British and Danes? Because it was a culture sharing, not a rape or pillar the way the other two did.
The native American converts ended up learning Spanish, it doesn't mean they lost their traditions or anything though.
The language of a community forms an important part of that community. For example, there are some things that can be said in one language that cannot be said in another. When a language is forced on a people, the cultural identity and national identity can become eroded as children then do not learn the language of their parents and grandparents.
I believe a nation's national identity can be lost by forcing a language upon it. I think if you look at places like Quebec who are really identified by their language, they could easily lose their identity if English was forced upon them. I don't think most nations are like this, but there are a few examples.
When we look back at the horrible things that were done to the Native Americans by settlers and to the Scottish peoples by Great Britain (and India as well), we see that although languages were forced upon these areas, it did not cause a total loss of national identity. While India saw this to the least degree, Scotland to a lesser degree, most of the Native Americans had no written language. While most would see that as a severe disadvantage in a world forcing them to learn English, it is actually an advantage to a certain degree. The children being forced to learn English (the adults were not treated in the same manner, but still mistreated and discouraged greatly from using their native tongue at all by not being given business or work when not speaking English) could not be watched at all times and many memoirs tell the stories of Shoshoni, Apache, and even Cherokee children speaking their languages in private and teaching younger children and white children as well. Children are most receptive to language and if they clung to that part of their past while still being in a new generation that would naturally accept new and exciting prospects, then a nation’s identity is probably established in family bonding or that an identity when threatened protects itself with natural desire from its peoples to be seen and heard, a sort of defense mechanism at the social level.
No, a nation's national identity cannot be lost by forcing a language upon it, because it is still the same nation. The United States would not have turned out very different if our national language had been German, as it was close to being at one point. We still would have been a great nation.