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.
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.
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.
In the sixth season, several characters lament not having enough tea on hand. Although not spelled out in the show this is because of the ongoing imperial crisis, which by the early 1770s was manifesting itself in new imperial regulations on tea importation. To combat these regulations, most colonists agreed not to import tea, or at least not to do so through legitimate sources.