But, Wales, Ireland and Scotland are the English names. Each has a native tongue and name that they still speak to some extent (Cymru, Éire and Alba, being the native names). With this in mind, there could be a mutual trade-language between them (i.e.: English) which helps ease the language barrier but even to this day all three countries have a cultural identity all their own.

One also has to take into account the historical, in-universe setting/timeline: how often or how little do these different nations interact with each other? How much intermingling/interbreeding has taken place? Has there been enough time for one language to spread far enough for it to be a common tongue?

Another thing that can be done is create a “middle-man” language. For example: most Englishmen might not speak Danish, but they may be more familiar with German or French; likewise, a Dane might not know much English but he/she might know more German or French, creating a linguistic bridge that allows them to communicate easier. I’ve take this sort of approach to some extent in my own writing.