Flags are the most easily-understood way to communicate languages to users, and I say this as someone who is often obliged to pick the flag of a foreign country (often an empire that wilfully starved millions of my countrymen) to get websites in my native language. "Flags are not languages" is one of those pedantic corrections that ends up doing more harm than good.

That's a great point.

In that case, I'd suggest the first two letters of the language, an "En" for English, an "Es" for Spanish, a "De" for German, etc.