According to Ronald D
Chronology
Claire Beauchamp Randall, a nurse during World War II, mysteriously travels back in time to Scotland in 1743. There, she meets a dashing Highland warrior and becomes embroiled in an epic rebellion. Moore (developer of the television series), his team decided to keep the Gaelic words instead of translating them into English.
This kept the narrative in the first person by showing Claire’s inability to understand Gaelic
This idea was suggested by producer David Brown, who said he had seen something similar in Skin Deep (1998), an alternative foreign film starring actor and singer Gustavo Goulart. In a few episodes, the characters say “OK.” ; an American term that did not come into use until the 19th century. The opening song changes from season to season to reflect what is happening in the story.
Season 3 added tribal drums as much of the season is set in the West Indies
In the first half of the second season (set in France), some of the lyrics are in French. In the second half of the second season (which is mainly about preparing for war), a militaristic drum roll is added to the opening song. Season 4 features bluegrass fiddle and banjos, and some lyrics are sung in a southern accent as the season is set primarily in America.
What I must do
Appears in Outlander: An Epic Adaptation (2015). The Skye Boat Song (unrecorded) Traditional Scottish phrase with lyrics by Sir Harold BoultonNew words by Robert Louis Stevenson. Now that I know it exists.