Mutual Intelligibility between English and Scots
Frisian is often cited as the language that is closest to English, but Scots is actually closer (i.e. has a higher degree of mutual intelligibility with English). Not Scottish English, which is a variety of English, or Scottish Gaelic, which is actually a Celtic rather than a Germanic language, but Lowland Scots.
There are just over 100,000 native speakers and it is classified as a traditional language by the Scottish government and a regional or minority language by the European Charter for Regional or Minority Languages.
Here is a lecture in Scots about the history of the Scots language. How much can you understand?