Ian has given you a great answer. I'll just add that Scottish Gaelic is in the Celtic branch (which includes Irish Gaelic, Welsh, and Breton, for example) of the European languages, unlike English (and Scots) which are in the Germanic branch. Other members of the Germanic branch include High German (Deutsch), Dutch (Netherlandish / Hollansk, Afrikaans, Flemish).
Nowadays, most people in Scotland speak English with more or less of a Scottish accent, as well as some regional typical vocabulary. A "thick" Scottish accent can be difficult (or impossible) for other native English speakers to understand.