I wrote a diary entry about it at https://www.openstreetmap.org/user/SomeoneElse/diary/42069. Since then I updated the Welsh processing to use a polyline: https://github.com/SomeoneElseOSM/SomeoneElse-style/blob/master/welsh_areas.poly. The Scots Gaelic area is still a rectangle.

This has been attempted, see https://lists.openstreetmap.org/pipermail/tagging/2018-April/thread.html#35855, but certainly wasn’t universally accepted as a good idea.

Another reason why that wouldn’t work for me is that there isn’t a strict isogloss between English and Welsh (or the others that I use). It’s a percentage game (see e.g. https://en.wikipedia.org/wiki/Welsh_language#Wales) and there are an infinite number of areas that you could draw, each corresponding to a particular percentage in a particular study, and of course other studies might ask different questions and get different answers.

With a DWG hat on I get lots of language questions, and often the “languages that people speak”, the local “official languages” and the “languages used when they want to communicate with officials” are very different - often there’s no simple answer to the “what language should I show” question.