Linguistic and cultural fluency requirements
The linguistic assimilation problem can be addressed partially by requiring, or strengthening requirements for, linguistic and cultural fluency as part of the condition to migrate. For instance, the United States might impose a test of English language fluency on immigrants. What precisely the standards of fluency need to be is a moot point, but a reasonable criterion would not go higher than the median of the country admitting the immigrants. Possibly, the minimum fluency required would be far lower than the native median. Such linguistic fluency requirements would be akin to the competency requirements for a driver’s license — checking that a person’s language knowledge is not so poor that the person poses a significant threat or inconvenience to others.
For those who believe that, in addition to language skills, some cultural literacy is also necessary for success, the linguistic fluency requirement may be supplemented by a cultural fluency requirement. Again, the cut-off should not be higher than the median of the country receiving the immigrants, and it may well be much lower.
Secondary effect of clear linguistic fluency requirements
If people have a realistic chance of being able to immigrate if they meet specific linguistic fluency requirements, this benefits not only those who already possess the necessary fluency, but also others who now have a strong incentive to train for it. If the possibility of immigrating to the United States, United Kingdom, or Canada becomes realistic for more people, they may invest in learning English. In countries where English is not a commonly spoken language, many people flock to English language academies to acquire the linguistic fluency that helps them trade with and work for people in other countries. English language academies would attract even more people if the possibility of immigration is realistic.
The same point applies, with lesser force, to the dominant languages in other countries that may be targets of immigration.