In the aftermath of WWI the newly-united Poland had a military equipped with a mishmash of leftover light machine guns, from Chauchats to MG 08/15s. They wanted to adopt a new standardized weapon, and trials in the 1920s found the FN BAR to be the best option. Unlike the American military BAR, the FN version adopted by the Poles used a light bipod and a pistol grip for better handling. It was chambered in 8mm Mauser, which was the standard Polish cartridge. The gun was adopted as the wz.28, or Model 1938.
The purchase agreement with FN was to buy 10,000 guns outright and also a license for domestic Polish production at F.B. Radom. However, the deal went quite sour when it urned out that FN actually didn’t have the technical package to supply to the Poles, since they had not actually tooled up to make the guns, instead importing them from Colt in the US. The Polish military wound up reverse-engineering the Colt/FN guns to allow domestic production, and the incident put such a rift between FN and the Polish military that they would develop the Vis 35 “Radom” pistol in-house rather than license the FN High Power several years later.
Many thanks to my friend (and regular commenter here) Leszek for background information on the wz.28!