You seem to be approaching this backwards.

Swiss German is German. It gets a German flag. Swiss French is French. It gets a French flag.

As a Brit you get used to American flags to denote English. It's just not a big deal. I mean I even use US English most of the time as I prefer the keyboard layout.

Barely any languages exist in the world without an associated country. You're looking for a problem where none exists. Romansh has like 50,000 speakers and a ton of those probably use something different on their computers anyway. But even there the clear and obvious choice is the Swiss flag.

Back in my day we all had to use ASCII, etc etc.