In fact, part of a reasonable definition of a language as opposed to a "code" is that it has varieties suited to various contexts. The frequency of grammatical forms and lexical items varies by context fairly consistently.
Not everybody has a colloquial register, but most do. Not everybody has a formal, academic register, because that's highly artificial. It tends to reflect written norms--and remember, reading and writing are fairly new things for the human brain--and is very explicit.
When "regional" languages like Galician or most of the First People's languages or even widespread languages like Swahili or Hausa encounter science they have to innovate, make up or form words, for new things. They also tend to take what's relegated to the corners of spoken language and include them. Most people don't use a lot of hypotaxis in their speech.
I speak standard English, pretty much. My dialect natively uses the subjective and it drives me slightly crazy when people say "It's important that he's there tomorrow". I also have a strong distinction between "many" and "more", "fewer" and "less." I maintain the distinction in vowels between "Don" and "Dawn" lost in most varieties of English--almost nobody in California, for instance, or West of the Mississippi unless they have strong NE influence, has this distinction. On the other hand, I have a rhotic in "Washington", "wash," "water."
Had a friend whose English was fairly standard. Didn't have a rhotic "wash" but did say things like "this needs cleaned" instead of using the verbal noun "this needs cleaning."
And, no, you're not going to make speaking a specific form of English for citizens a requirement. For instance, I know African-Americans who didn't master "NBC English" and who'd fall under your sanction. (And, no, if I say it's "Ebonics" then there are going to be a whole lot of new languages formed from what used to be called English. Take "Scotts", considered by some to be a different language. Their primary argument is nationalist and separatism. So it is with "Ebonics". It's AAVE.)
"Standard" is a narrowed range of alternatives. And it reflects some sort of power relationship--one that anybody can join, but which tends to be required for admission. Nobody expects immigrants to master English. But more than a few times I've gone into a store or restaurant and had to order in a language other than English. In a deeply ethnic neighborhood, I could see this: your language limits you and you're stuck there, unable to advance or even be decently educated. But when it's not, all it means is that the employer really scraped the bottom of the barrel. I've been in other countries, and typically these "enlightened" countries have all pretty much believed that it's an insult to move there and after a year or five still not bother to learn the language or basic elements of culture. (It's rather like snooty British officers who'd go to India and insist everybody learn their language in a lot of ways. "You adapt to me, sucker." I feel sorry for immigrants who are here and don't learn the language. If only because of the kids I failed in the last couple of years, probably half of them were born here and really didn't want to learn English.)
Some will defend this when the language is Spanish. If it's Chinese or Russian, not to much.
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Sorry, no one gets in unless they already speak English. 
