The use of Scots in the dialogue of Kidnapped is notable.
While Sir Walter Scott used Scots in his historical novels, such as The Heart of Midlothian, Stevenson used it differently. Scots is the language spoken by servants and the lower orders. While sometimes these characters are quite important, such as Jeanie Deans, the powerful people, such as the Duke of Argyll, speak English.
Scots is used differently in Kidnapped. The main characters all speak Scots, or at least a mixture of Scots and English. Alan, for example, would have spoken Gaelic as his first language but may have learned both English and Scots at the same time.
Alan has to use all of them in his trade as a soldier and clan messenger, travelling through Scotland, England and France. But he speaks Scots at least as much as David and this is a distinctive trait Stevenson uses to give him depth as a character. His manner of talking helps to make him sound even sharper and tougher.
David uses both Scots and English. The narrative is by him and is all in English. What is interesting to notice is when he uses Scots in the dialogue - often when his emotions are roused and when he wants to make a point.
Language matters in other ways too. In his meeting with Rankeillor, David shows he understands Latin and this helps him, as Rankeillor uses Latin as a kind of test to see if David is who he claims to be.
The significance here is that Latin is a language of learning and education, and David is aware of this and eager to pass the test Rankeillor sets him.
David is always conscious of the disadvantage of not knowing Gaelic and this feeling is acute at some points in the story. Gaelic is not represented in the novel as the other languages are, but the reader is made aware of the fact it was commonly used across the Highlands at this time.
What is perhaps less obvious to us, but still very important, is that Scots was just beginning to lose its place at this time. The Union of Parliaments was less than 50 years earlier and there were still plenty of areas in which Scots was the official language, such as in burgh records.
However, English was steadily taking over and we can see the gradual process whereby a Scots word would be replaced by an English one. But we still sometimes use Scots words today in official contexts, without realising they are not English, eg 'outwith', and 'leet'.
The fact remains that Kidnapped shows us a Scotland in which English is becoming the language of power, but where those who also speak and understand Scots and Gaelic get around more easily.