Character mistake: Scarlett says that since McCullen is from Scotland, he speaks Celtic. Celtic is a group of languages, not a language unto itself (something a genius like Scarlett would most likely know); it would be like saying someone speaks Semitic or Germanic. McCullen would probably speak Scots or Scottish Gaelic, the official languages used in Scotland.
Character mistake: Stormshadow is a Japanese American, but in Snake-Eyes's flashback set in Tokyo, Japan 20 years earlier, Stormshadow calls out "Do dook nom ee dah," which is Korean translated to "Thief!" when he spots Snake-Eyes. (00:43:25)
Suggested correction: Traditionally Stormshadow is Japanese American. In this film he is played as a child by Brandon Soo Hoo (a Chinese/American actor) and as an adult by Byung-Hun Lee (a Korean actor). Although it is never stated, it is clear based on the fact that the young actor speaks Korean and as an adult is played by a famous Korean actor, Stormshadow in this film is intended to be of Korean heritage.
Suggested correction: Scarlett certainly took this into account when she suggested the words for Ripcord to speak. Scottish Gaelic is a Celtic language so Scarlett was still correct, she was just simplifying for expediency.
Phixius ★
No she wasn't correct. There is no such language as "celtic."