The textbook answer is that it's a dialect if 2 persons of good will and reasonable sense are able to understand each other. Mostly. Years ago, PBS ran a series on the English language and it's dialects. They let the Lowland Scot faction call their dialect a language (clearly incorrect because they were understandable. Mostly) and added that Scots Gaelic was the native tongue of one or two outer islands only. The dialect and the Gaelic were plainly unrelated, except perhaps for loan words.