I actually think that if there are not enough natural inducements to learn the common language of a country, and you have to introduce linguistic requirements, then it is a clear indication EITHER that your society does not allow immigrants to intergrate properly, or that the imigrants are willfully isolating themselves from society for their own cultural reasons (say, religious).
If an immigrant knows he won't be accepted easily into the main-run of society due to his background, and that maybe he can do better just mixing with his 'own kind', he won't have to be fully proficient in the common language of the country.
If this happens IT IS THE COUNTRIES FAULT. Making a law to force the immigrant to learn the common language won't remove the factors that make him feel like he's better of mixing with his own kind. What has to happen is positive action to make the immigrant community feel part of their adopted country.
On the other hand, if an immigrant group is deliberately isolating themselves for cultural reasons, then I think it is THE IMMIGRANTS FAULT. It also means that, in addition to not speaking the language, children growing up in that environment will not be bought up the same way as their contemporaries in the larger surrounding culture, which can often mean they are sexually disadvantaged as well as linguistically disenfranchised. I think that is as bad as an immigrant community feeling so discriminated against they elect not to take part in their surrounding culture.
Essentially, making the immigrant feel it is to his advantage to learn the common language works, and will seldom require legislation or compulsion.
One also has to recognise that sometimes fluency won't happen... yes, maybe a 70 year-old illterate female peasant from Bangladesh COULD learn English fluently, but to impose the same expected standards of fluency upon her as a 25 year-old with a normal modern education is not reasonable.
People living in glass paradigms shouldn't throw stones...