Teaching the history of the UK, including that of the different countries within it and the fact that different areas has different languages and cultures etc is something I agree with. I don’t think that should extend so far as teaching all of the languages though, for a start there are unlikely to be enough teachers who have a good enough grasp of the language (including accent and pronunciation considering there are lots of sounds used in Celtic languages not used in English) and I don’t think teaching kids in London a few words in Welsh, for example, will stop it dying out.
To stop languages dying out the focus needs to be on the areas where the language still exists and is still used. Yes, in areas where a language is part of the cultural history and where there are still some native speakers I agree with teaching it in schools. But I think supporting a language not to die out goes beyond teaching it as a second language. It’s about ensuring that the language is used in things like television programmes, literature, newspapers and magazines, translations of government documents, road signs etc. If a language isn’t useful and something that can be practised in day to day life almost nobody is going to see a value in learning or speaking it.
Wales have really focused on keeping the Welsh language alive in recent decades, it’s compulsorary to learn in all schools, all road signs must have Welsh alongside English, publications from services such as the NHS must be available in Welsh as well as English. When I was in school in the 90s almost all schools in my local area were English medium but now the majority of primary schools at least are Welsh medium. There also seems to be a revival of giving children Welsh names, again when I was in school only the children from families where Welsh was the first language tended to have Welsh names but now they seem more popular among English speaking Welsh people too. When my generation was at school Welsh language speaking was really promoted and I think this has seen the value of the language improve and the percentage of people in Wales who can speak Welsh has mostly been increasing over the last couple of decades. Promoting a language in the area where it has cultural significance works but I’m not sure that trying to revive languages in parts of the UK they’ve no historical significance would be very effective.