If you like flags because they are colorful and memorable icons, you can use them to represent time zones. Much more convenient than some obscure "UTC+7" or "PT", isn’t it?
Because a currency symbol is less correct (think $) and isn't as recognisable. Flags have colours that make them easier to pick out, and can be displayed at much smaller resolutions.
(Re: colors: see my suggestion to use these colorful icons to represent time zones)
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.
How many flags would you use for Spanish? French?
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?