There are certain sayings, phrases or slang that I may not be intimately familiar with.
This says nothing about your fluency. There are tonnes of English slang that Americans are unfamiliar with, and vice versa.
Hell, there's a lot of Singaporean English that doesn't exist in the minds of Brits and Americans, but Singaporeans are still fluent in English, it's just different from what people consider "true" English.
Had to look it up, it's close. Wiki says the foundations were laid in 1785, so not long after the US independence.
https://en.wikipedia.org/wiki/Jaeger_Corps_(Denmark)