English has a briton-latin base upon which numerous other languages have contributed such as saeson and norman french for two examples. It continually evolves because it is part of a root culture that practices an old celtic concept of pursuit of excellence, which means to strive to attain for oneself and ones people knowledge in all things. If a word or concept in another language does not exist in English at the time of encounter then English willingly accepts its inclusion. A new word might take on a slightly altered pronounciation or character due to improper translation or perhaps a melding in a greater meaning or even a misinterpretation of the original.
Regional dialects occur when there is less interaction on a broader scale and linguistic or pronounciation habit asserts a common usage.
Spelling differences can occur during times of locational transition because that was the accepted method of that time, yet the parent source continues its own evolutionary path and thus deviated from that point while the child group retains the original. And sometimes the reverse can be true.
Study of the linguistic peculiarities of English rapidly becomes a trip through history, culture, and geography.