Short answers, pulled from the story.
Old English emerged from a dialect continuum spoken by the Angles, Saxons, and Jutes along the coasts of Frisia and Lower Saxony. By the 7th century, this language had become dominant in Britain, replacing the Common Brittonic and British Latin that had been spoken during the Roman occupation.
The Great Vowel Shift was a chain shift that affected the stressed long vowels of Middle English and fundamentally altered the sound of the language. This shift explains many irregularities in spelling since English retains many spellings from Middle English, and it also explains why English vowel letters have very different pronunciations from the same letters in other languages.
The countries with the most native English speakers are, in descending order, the United States with at least 231 million and the United Kingdom with 60 million. Canada has 19 million native speakers and Australia has at least 17 million native speakers.
In 1755, Samuel Johnson published his Dictionary of the English Language, which introduced standard spellings of words and usage norms. This publication helped establish explicit norms for standard usage that were spread through official media such as public education and state-sponsored publications.
Braj Kachru has categorized countries into the Three Circles of English model, according to how the language historically spread in each country, how it is acquired by the populace, and the range of uses it has. Inner-circle countries have large communities of native English speakers, including the United Kingdom, the United States, Australia, Canada, Ireland, and New Zealand.
Parity with French as a language of diplomacy had been achieved by Treaty of Versailles negotiations in 1919, and by the time the United Nations was founded at the end of World War II, English had become pre-eminent. It is one of six official languages of the United Nations and is now the main worldwide language of diplomacy and international relations.