Many languages don't correspond to flag ideograms, and many flags don't correspond to a single language.

Ideograms are fine so long as they can be replaced with a canonical word or phrase and still make sense.

Flags for languages fails that in many cases around the world- if I want to select Quebecoise French, neither the Canadian, French, or Quebec Province flags are appropriate.

Likewise, Switzerland has no official language, and many countries such as Belgium have multiple.

Not the GP, but ideograms/pictograms are overused, half of the time one has no idea what they are supposed to mean. That was already bad in desktop interfaces where you would at least get a tooltip when hovering the mouse over it, but that has gone out of fashion as well.