Sure, fine for personal uses. I mean broadly and generally.
As for English, the United States has far and away the largest number of native English speakers.
Not that I think the stars and stripes has any more right to represent “English” as a concept any more than the Union Jack. If you’re going on origin, why not the flag of England instead?
I mostly meant that facetiously as now we're entering the linguistic quagmire of trying to pin down an exact origin for a language, and furthermore (depending on your chosen definition of "English") the language itself predates the current flag of England, so even that is open to debate regarding its appropriateness.
The moral is: don't try to draw boxes around languages.
All that said, I do understand why someone would want to use flags as shorthand for language. It's wrong, but it's useful.