Scottish Gaelic Language
The Scottish Gaelic language arrived in Scotland with the early Irish settlers on the west coast around the 4th century. For centuries, the two languages were virtually identical, being used by the Celtic Christians from Ireland who evangelized Scotland and traders between the two countries.
Over time, however, as Irish and Scottish history and government diverged, Scottish Gaelic began evolving in its own unique way. As the western Scots settlers gained ground in Scotland, defeating the Picts and merging with them, the Scottish Gaelic language spread throughout Scotland and remained in use through much of the country for centuries.
After the Norman invasion (11th century) began affecting Scotland, with the French invaders and their English armies and retainers moving up into Scotland, a Scottish form of English began developing in southern Scotland. Eventually, Scotland was linguistically divided between the English-speaking lowlands and the Gaelic-speaking Highlands and Islands.
Today, fewer than 60,000 people speak Scottish Gaelic, mostly in the Hebrides and other islands, though an enthusiastic project has been ongoing for some years to revive and preserve the language. More and more Scottish young people are working to learn Gaelic.
Scottish Gaelic is one of six celtic languages still in use. The others are Irish Gaelic, Welsh, Manx (Isle of Man), Breton (Brittany in France), and Cornish (Cornwall in southwest England). There are also dialects of Gaelic spoken in Cape Breton, Canada (Scots Gaelic), and Patagonia, Argentina (Welsh). All these languages are descended from the Indo-European family of languages, specifically the Proto-Celtic subgroup of Indo-European languages. Scottish Gaelic is further descended from the Goidelic branch of the Celtic languages, as are Irish Gaelic and Manx. Both Scottish and Irish Gaelic developed from Old Irish.
So it goes likes this: Indo-European > Proto-Celtic > Goidelic > Old Irish > Middle Irish > Scots Gaelic.
Originally, the ancient Irish wrote with the Ogham script. When the Irish converted to Christianity, the monks began using the Latin alphabet, substituting them for the Ogham letters. Because of this, written Scottish Gaelic uses only 18 letters, each of which is named for a tree or bush - harking back in time to the ancient Druids and their reverence for nature and forests.
As you know, if you have tried to read Gaelic online or in a book, pronunciation is virtually impossible to figure out. I've added a few books and dictionaries here for those who want to learn, but I'm sure a spoken Scottish Gaelic language software program would be indispensable in learning Scottish Gaelic.