For a start Scots is a dialect or language variant of English. Welsh and Cornish are NOT Gaelic languages, but Brythionic languages. The 5 you've listed are together Celtic languages. There's quite a big linguistic difference between the two groups.
In the UK there are a number of indigenous languages, in particular the UK has three living and "official" languages: Scots Gaelic, Welsh and English. Irish in Northern Ireland and Cornish have a slightly different statuses.
Where do we stop? Well, there's a common sense answer which is to use those languages which are official and living in those areas to service to/from those areas. So on trains in and to/from Wales I'd expect to hear Welsh and English; similarly on Scottish services, though that poses a different problem in that Scots Gaelic is much more confined to certain areas than Welsh is for example. There really is no problem with multiple language usage and if GWR wish to use Cornish on services to Cornwall as a method of promotion then that's fine. Similarly use of Scots on lowland Scottish services if Scotrail or whoever so wish.
Use of other languages can be considered for certain purposes, eg: Latin and Urdu have been used on railway signage, but the cases here are very specific-
But the Gaelic thing is a bit of a nonsense. It is fine going up the West Highland line but its introduction into south west Scotland just shows how ignorant Transport Scotland are. The Gaelic that was spoken here was a totally different dialect from that in the Western Isles. It has more in common with Manx and Ulster Irish. We had Galwegian Gaelic as opposed to Highland/Scottish Gaelic. The last native Gaelic speaker died out in 1760. On that basis we might as well have bi-lingual signs in French.
This is the trouble with a nationalist Government up here. They are trying to push anything that is anti-English just as in the Republic of Ireland, where I do wonder why they changed to kms? I was hoping they might also change the side of the road they drive on and just await the chaos at border crossings....
In Scotland a more appropriate bi-lingual language would be Scots and English. Mind given the sometimes incomprehensible nature of some of our inner centre brethren I'm not sure sub-titles might not be more useful.
A few 'corkers' for those not used to the different tones up here.
Generally in Scotland there is no letter J (jay) they pronounce it 'Ji' as in die
Irvine on the ScotRail announcements comes out as Oven and Ayr as Ear.
The rest is mainly dialect and here there is quite a similarity with geordie. So if you've liived in Newcastle the transition 'alang d' rudd' shouldn't be too much of a problem.
My personal favourite is Carlisle. Southerners say Car-ly-ul, Scots say Cu-ly-ul and Cumbrians Carr-lile. Me I'm with the Cumbrians.
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No need to refer to Gaelic as Scottish Gaelic. There's no other Gaelic. Irish is Gaeilge, often referred to mistakenly as Gaelic.
Sorry, I'm splitting hairs now. I'm all for bi-lingual signs, regardless of the language. As long as English is on it, what's the issue, politics aside?
You are wrong there. Also spoken in the Highands it is pronounced Gall-ic, (as in galley) in Ireland it is Gay-lic.