National flags are not in general a very good way of labelling languages. There are far more languages in the world than countries. In any case, since people are looking for a language they understand it's good enough to write each language name in the corresponding language, like Wikipedia does (wikipedia.org).
But that's not the question, anyway. The question is how to label the button that lets the user change the language when the user might not understand the current language at all. Perhaps a big bright "?" ...?
National flags are actually a very good way of labeling languages for two reasons: first and most important, almost everybody already understands that a flag-looking icon (or two-flags-stacked-on-each-other icon) is used for switching languages. That's already a very strong practical reason to use them. Second, for like 70% of the most popular languages there are flag assignments that won't mortally offend the speakers of those languages — maybe they'd rather see a different flag but generally they'd grumpily agree that "guess it conveys the intent good enough, whatever, I've managed to chose the actual language I want to use".
