We speak English together all the time, but we are both fluent in French and switch whenever we are in company.
OH will always have a strong English accent, that everyone loves here- but he can communicate freely. He is a scientist and only skimmed his O'Level and is certainly not a natural linguist. But it was always clear to him that if he was going to spend lenghthy periods of time abroad, he had to learn the language- out of respect for the locals. Very few anglo-saxon immigrants here, but plenty from Italy, Portugal, Spain and the Balkans.
Boston and East Europeans, or other towns with other immigrants- changing the culture of those areas - yes, this has always been so, Picts, Jutes, Angles, Saxons, Romans, Normans - and so many more over the centuries. What I strongly object to, is that often the very people who complain about the above, go on and do exactly the same, even 'worse' as they don't even learn the local language at all. And THEY do change the culture. Very many notable and wonderful exceptions, I know.
You can go to some towns in the South of Spain, Cyprus, in parts of France- where the local language is not heard at all- and shops and restaurants, markets, etc- cater almost 100% to those incomers.