How many flags would you use for English? If you want to use flags of the most populous countries where English is the primary language, the second country after the US is Nigeria (182 m people).
Well, how many languages have flags? There are thousands of languages, but hundreds of countries. Surely, some languages would be left without a flag?
And this doesn’t affect just some "obscure" languages. For instance, what flag would you use for Arabic, an official UN language? What language does a flag of India, most populous language stand for?
If you have only one version per language, just show the flags side by side. UK/US/AU/NZ for english, DE/AT/CH for German, etc.
On the other hand € stands for 20+ languages and $ means USD, NZD, CAD, many kinds of Peso and many other types of dollar.