When did English become the dominant language of England, and why?

Same question about France and Germany. Many languages were spoken by relatively isolated groups in Europe, yet one came to dominate. Are there any books on this subject?

A subset of the above question: How many countries besides the USA were essentially monolingual for any significant time? I do understand that many languages have always been spoken in the USA, but only recently have they been officially acknowledged.

Well, it depends what you mean by England and dominant. But seriously, any good book on historical linguistics and/or socio-linguistics should do the job. There are a bunch of books specifically about the history of English that could help, too. In the historical period in Great Britain (i.e., when writing existed), Brythonic Celtic dialects (i.e., the ancestral languages of Welsh, Breton, and Cornish) were spoken. Then the Romans came and made Latin the default language of government. When the Romans left, Angle and Saxon tribes took the opportunity to invite themselves (the first people showed up as mercenaries for some of the Romano-Celtic kings) ca.500 CE, and pushed the Celts into the fringes (i.e., Cornwall, Wales, and Scotland). Later the Irish (a different kind of Celtic started raiding Wales, the Isle ofg Man, and Scotlanf). The next big change (ca.1066 CE) came in when the descendants of Vikings who had lost Old Norse and picked up Old French invaded from across the channel. Vikings direct from Scandinavia invaded the area around York. Norman French and Middle English competed for a couple of hundred years or so, but soon English won out. Scots (i.e., the non-Celtic variety of English spoken at the Lowland royal court started to replace Scotch Gaelic in this period, too. Irish had developed into a different language by this time (Scotch Gaelic) and replaced Brythonic Celtic in the Lowlands and Pictish elsewhere. Many dialects of English were (are still) spoken in various parts of England, but the dialect round about London became the default standard in the Early Modern English period (ca.16th century).

Basically, for the rest of Europe, privileged dialects (usually spoken by whomever was running the country) became the standard languages. In Germany and Italy this took place late, in the 19th century. (In fact many of my ancestors immigrated before Tuscan Italian became the "official" language of Italy.) In France a little earlier. Other languages, e.g., Catalan, Basque, Breton (an import from Celts fleeing England), Provençal, Sorbian (in eastern germany), Polabnian (in Poland), etc.

As for the USA, there were literally hundreds of languages spoken by the indigenous people when the colonists started showing up. In the colonial period, Dutch, German, English, Spanish, etc. were spoken. In small pockets, Basque, Welsh, etc. were also spoken.