For this reason, I think that migrants should be allowed to conserve their own language and to create communities in which their culture can be "rebuild" and where they can feel like "at home".
By allowing this, the host countries would help the migrants to adapt easier. But efforts should not come only from one side, migrants should also respect the new "home-countries" and their cultures. And, in my opinion, one form of respect is to learn the local language. What ever the reason was to migrate, the immigrants still are in a foreigner country, for this reason they have to adapt to it and not just continue to live their previous lives without paying attention to the people by which they are surrounded.
In conclusion, migrants should be able to conserve their languages and cultures but they also have to make some efforts to integrate in the new country.