The Hindi argument is not good, Hindi is the _official_ language of India. I work with people from various parts of India and they can all communicate with each other because thry all speak Hindu/English (usually a mixture in conversation), even if their main language is Malayalam.
There are issues with flags = languages (e.g. "Welsh": "Gaelic" for instance, or Switzerland (which one?), but for English/Spanish examples, they are not good ones to use
And to end this, just because there is a website for your issues doesn't mean it's correct. There is nothing preventing someone from making a website called flagsarelanguages.com and having poo counter points to the original websites poo arguments. Appeal to authority nonsense.