It's used commonly in English. It turns up in both US and UK (and Australian... etc...) styles.

"knuckle down" is used in American English. I have to knuckle down to learn English. Or: I have to knuckle down and learn English. In his new office, he suddenly had to knuckle down to work. Or: In his new office, he suddenly had to knuckle down and do his work. [If you place "in his new office" at the end of the sentences, it sounds like the important thing is where he has to work, rather than the fact he had to knuckle down.]

I don't know it's used in in America. Maybe in the older generations. I know we say "buckle down" to mean the same thing Example: "Time to buckle down and study." However we do use the term "knuckle up" to mean "prepare to fight" Example: "Knuckle up, it's time to fight."