Is there a way to use a US and MEXICO flag for English and Spanish? The UK and Spain flags don’t represent our customers how we would like.

Yes! You can do it from that page: https://dashboard.weglot.com/settings/language-switcher.
Checkout our document over there: https://support.weglot.com/article/280-how-can-i-change-the-flags-of-the-language-switcher.
Best,