According to Ronald D. Moore (the TV Series developer), his crew decided to keep the Gaelic words instead of translating them to English. This maintained the first person narrative by showing Claire's inability to understand Gaelic. This idea was suggested by producer David Brown, who said he's seen something similar in Skin Deep (1998), a foreign alternative movie starring the actor and singer Gustavo Goulart.
The actors portraying the 18th century Scots did a sort of boot camp to learn Gaelic and make sure it was word perfect. Ironically the one person who knew how to speak Gaelic is Caitriona Balfe who plays Clare who does not know how to speak it.
With the renewal for season 8, it became the longest show in both episodes (101) and seasons of all of Starz's history.
Many of the actors portraying Scots admit they wear their kilts authentically, meaning no underwear is worn beneath them.
Caitriona Balfe, who was a ramp model before she ventured into acting, said in an interview that she is used to getting naked in front of 30 people and is confident in her body.