Counterpoint to this website: it is convenient to have symbols to represent things. I don't see them suggesting anything better than flags.
People have proposed "language flags" before. In fact, Wikipedia even uses/maintains a set of them for Wiktionary entries ( https://en.wiktionary.org/wiki/Wiktionary:Language_flags_lis... ) — although I've never personally noticed them in use in Wiktionary. (Does anyone have an example of a Wiktionary page that embeds one of these?)
Perhaps “language symbols”, which take the idea of language flags and simplifies them into colored glyphs that render nicely at small sizes could be a solution.