Q What OS X and browser support is there for Scottish Gaelic and other Celtic tongues?
A OS X provides limited support for Welsh and Irish, with localised keyboard layouts available in Yosemite, but this does not extend to localised versions of any of Apple’s major applications, such as Safari, in those languages.
Andrew Carnie provides a localised Scottish Gaelic keyboard here.
The two browsers with the most extensive support for even relatively little-spoken tongues are Firefox and Opera. There are fully-localised versions of Firefox available for Welsh, Irish, and Scottish Gaelic, and a general listing of localisations. If you fancy grappling with Breton, try this although this may not offer your preferred dialect or orthography! Other Mozilla products enjoy almost as broad support too.
Opera ships with limited localisations built in, but you can add Scottish Gaelic from extra files available here; it does not yet appear to support any of the other Gaelic languages.
Spell-checking support used to be more of a problem. The old method of installing cocoAspell, and using any of the huge range of dictionaries available for aspell by ftp from ftp.gnu.org/gnu/aspell/dict/, has been superceded by Sealgar’s dictionaries.
The Scottish Parliament has a good page of other links.
Updated from the original, which was first published in MacUser volume 27 issue 17, 2011.