Earlier quoted context omitted.
You didn't answer the question. The claim was "countries that are based on a unique language, ethnicity and culture, [immigration] is damaging." Again, I ask the question: damaging in what sense?
If you accept that the language, ethnicity, and culture are valuable parts of that country- Such a country can assimilate some number of immigrants each year while remaining mostly unchanged. The immigrants learn the language, the culture, and marry into native families. But as you increase the number of immigrants, the country can no longer assimilate them. They don't all learn the language, they don't know the cult…
Things change. That's called life. To assume all nations and all cultures remain static over time is absurd. Hell, the nations as they are defined today are a very recent phenomenon. Some have only existed for a few decades!
The host country will become a new, different, more diverse country. That doesn't mean the culture of the host country is completely destroyed. Altered, yes. Destroyed? Almost certainly not.
And the detrimental effects can be mitigated if the host country works to actively integrate (note, integrate, not necessarily assimilate) immigrant populations, rather than isolating them through xenophobic policy and politics. It's the very fear of change that exacerbates the challenges of immigration.
As an aside, your characterization of immigrants as from "Elbonia" coming to make a "little Elbonia" is precisely the kind of xenophobic tendency I'm talking about. I'm sure folks said the same thing when Little Italy or China Town showed up in New York City, and yet today those areas are considered cultural jewels, contributions to society rather than infections that must be dealt with, components that have been integrated into a modern American identity.
Maybe don't assume immigrants are foreign invaders and they'll be more likely to integrate rather than isolate...