Though the vast majority of Scots speak English, to the untutored ear the Scottish dialect can be hard to understand, as many words and expressions are derived not from English but from Lowland Scots, or lallans, which is now recognized as a separate language as opposed to simply a regional dialect. In the Highlands and Islands, the accent is very clear and easy to understand. In fact, it is said that people from Inverness speak the clearest English in the UK.
Scotland's oldest surviving language is Scottish Gaelic (Gaidhlig, pronounced 'Gallic'). Often referred to as the national language, it has been spoken the longest. Introduced to the country by Irish immigrants in the third and fourth centuries, its use soon spread and became well established. The language is spoken by about 85,000 people in Scotland (about 2% of the population). This is in the Gaidhealtachd, the Gaelic-speaking areas of the Outer Hebrides, parts of Skye, and a few of the smaller Hebridean islands. Gaelic is one of the Celtic languages, which has included Irish Gaelic, Manx, Welsh, Cornish and Breton. Today only Scottish and Irish Gaelic, Welsh and Breton survive.
Those wishing to teach themselves Gaelic could start with the BBC CanSeo cassette and book. A good phrasebook is Everyday Gaelic by Morag MacNeill (Gairm). Also, the Celtic Heritage Centre, based on Arran, has a great store of Gaelic and Celtic information and documents; they publish a quarterly newsletter. Macbain's Etymological Dictionary of the Gaelic language contains a wealth of words and information.