For immigrants, is it a violation of human rights to use cultural assimilation policies ?
Not necessarily. Your question doesn't take into consideration that assimilation is either willful, an act of volition, or coercive.
Take for example, nineteenth century schools for indigenous Native American / First Nations children in North America. These schools robbed children of their cultural identities, heritage, and history, separated them their families, and Christianized their names. This is an example of coercive assimilation.
Conversely, as an anecdote from my family historian, one set of great-grandparents were first generation US citizens whose parents immigrated from Sicily. Their names were Guisippe and Francesca, but chose to go "John" and "Francis" to become "American" in an environment, which at the time, was hostile to immigrants from Sicily and Southern Italy. Hopefully this illustrates willful assimilation.
I don't think immigrants have any particular right except the one granted to human beings in general. There is no such thing as a right of immigration, unless you want to construe the rights of refugees as such1. But that would be a controversial statement.
There are rules that forbids the suppression of the culture of any human being, which of course include immigrants:
Everyone has the right freely to participate in the cultural life of the community [...]
- Article 27 of Universal Declaration of Human Rights
And there is also the historical precedent of an agrement that protect that rights of minorities: Gruber–De Gasperi Agreement. This agreement between Italy and Austria, after World War II, guaranteed the protection of the German and Ladin languages in the Italian region of Alto Adige. But these minorities were not immigrants, instead they lived there for a long time and this was an agreement between two weak states that followed a period of great repression. There are not such rules for minorities in, for instance, France which has a long history of centralization, both cultural and administrative.
Furthermore, as bilbo_pingouin has pointed out, not giving the chance of cultural assimilation to the immigrants would actually be against their interest. There are statistics that shows that less integrated immigrants feels more unsatisfied and are poorer. Nobody want to feel excluded from their new home. Which also means that, in practical terms, first generation immigrants prefer that their children learn the local culture.
In summary, there are rules and a recent history, in Europe, of protecting the positive right of everybody, including immigrants, of cultivating their own culture, but there are no rules that forbids initiatives to increase the cultural assimilations of immigrants. Or put in other terms, as long nobody forbids the immigrants to practice their own culture, everything is fine.
- since refugees are, by definition, outside their home country
To address the question from another angle, it seems clear that international law and human right treaties (non-binding as they often are) tend to deliberately avoid including migrants in texts protecting minorities. For example, the European Charter for Regional or Minority Languages explicitly excludes the “languages of migrants” from its scope.
Furthermore, sociology traditionally makes a distinction between “integration” and “assimilation”… but the more assimilationist countries or thinkers often use the word “integration” to describe their position. That is, no matter how strong the pressure is to assimilate, you can perhaps find some critical voices but there is no international consensus to consider this a major human rights problems and everybody just claims to encourage “integration”.