I think you are confusing language (English, French, Arabic, etc) with encoding (ASCII, UTF-8, UTF-16, Latin1, etc).
You do sometimes see mojibake in web pages, (question marks and □□ in place of the real text). These are caused by incorrect encodings. The web server tells the browser which encoding to use using the HTTP header Content-Type or using the <meta charset="UTF-8"> HTML element. You should always set the encoding rather relying on the browser guessing.
I was referencing asian languages as one set of examples for encoding. Of course setting the encoding is important (whether html or within a program dealing with strings - I had my share of hard bugs only to realize just that, another story)!
Here I was just sharing my encounters of the browser rendering ?? and □□ character marks, because it did not know. The browser had a "Auto Detect" charset mode, so one always had to toggle (and remember to revert back again when viewing another page). For the very reason you have indicated, "should always set", but in those days (and maybe even today), not always set.
You do sometimes see mojibake in web pages, (question marks and □□ in place of the real text). These are caused by incorrect encodings. The web server tells the browser which encoding to use using the HTTP header Content-Type or using the <meta charset="UTF-8"> HTML element. You should always set the encoding rather relying on the browser guessing.