Using flags as language "logos" is never a good idea. However, it works for a lot of cases and apart from that just looks cool. But when it comes to languages like English, it becomes difficult.
Hence, the (rather statistical) question: if you were to pick a flag for "English", which one would it be? Imagine your website is region-independent, i.e. you don't have grammar/vocabulary that is different across countries.
Ok, to be serious, I prefer the language abbreviations like EN, FR, etc.
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.