I wish to be able to write a text in both languages, highlight a piece of text in one language and spell check, toggling between spell check languages. I wish to keep all instructions on the page in UK English. Also, how can I add backward slash accents over letters?
Libre comes with a Gaelic dictionary, but doesn’t install it by default. You have to choose Custom install when that option is offered. See igaidhlig.net for how to install a Gaelic language pack, if you’d want that as well. Download just the dictionary from the extensions site.
The Scots Gaelic spell checking extension can be found at http://extensions.libreoffice.org/extension-center/an-dearbhair-beag-scottish-gaelic-spellchecker
In a dual language document you can use format->character->language to set the language of text to the non-default language. Once a region of text is set to Gaelic (Scotland) then it will be spellchecked with the above dictionary.
How to easily type the accented letters depends on the platform you are using. Under windows for example here’s a howto: http://www.igaidhlig.net/en/windows-sytems/