The ideal situation would be where every immigrant learns the language of the country he moves into, that's true. Unfortunately, we don't live in an ideal world. Just to warn you, this post will be very severely influenced by my living in a very poor region of the relatively poor country that Poland is.
We live in a country where the minimal wage is just about enough to pay for the bills of a run-down flat and the cheapest food. Thing is, it's growing increasingly difficult to find menial jobs where you can earn a minimal wage - we have the problem of so-called 'trash contracts', which essentially encompasses the gray zone of the job market; the employees are hired on a 'pay for creation' instead of 'pay for work' basis, which means the employers pretty much aren't legally bound to pay them. Or to pay for their health care, or to give them paid leave, or do anything that the Poles enjoyed throughout the years of being 'communistic'.
So, more and more Poles are forced to emmigrate in order to feed their families. Thing is, a ton of them comes from poor, uneducated families (vicious cycle, anyone?), so they never learned English nor did they ever feel the need to. When they get to England, which is quite often their first time outside of Poland (and the tickets have to be paid for as well), they find themselves surrounded by a completely foreign culture. Have you ever moved country? I understand it may be quite difficult for you, coming from a developed country, to empathise with problems of the Slavs, but, the English-speaking communities in Spain should be a tad bit more understandable for you - the easiest way out is to create a small enclave, where you can pretend you are still in your fatherland. We are all human, after all, you can't expect several hundred thousand people to willingly abandon their comfort zones.
And this comes to my final point: we're all human, we're all European, and we should strive for the entire humanity to have comfortable lives. Just closing yourself off from other cultures only speaks of closed-mindedness. That is to say, my country is suffering from that problem as well - a lot of Poles are racist towards the Romani, Romanians, Moldavians, Ukrainians, Kazakhs... The list goes on and on, and point to an irrational sense of possession: 'I was born here, so the land is mine'. There is never too little to share!
I don't expect my post to change the opinions of nationalists, conservatives, or anyone else who has despised immigrants before it. I just hope it makes you understand why we leave our fatherland and symphatise with our struggles a bit.