The US and Canada have the primary language of English. But Canada also has French, and certain sections of the US have Spanish as a predominant language. Just one example: If I have a US company that also ships to Canada, it makes sense to have the US flag for English and the French (or Canadian?) flag for French Canadian. It also makes sense to have the Mexican flag for the Spanish option, because most of my Spanish speaking customers are likely to hail from Mexico, with a small number from other Central American countries.

What would not make sense is to have a UK flag for American customers, a Spanish flag for Mexican customers, and the Democratic Republic of the Congo flag for French, just to appease people who will never buy from my site. You might not have national borders in mind, but it pays to know what language your customers speak as well as what countries they are from.