Slonim is Belarusian town. The town may had a Jewish majority, as many towns and cities in Belarus, Lithuania and Poland. Slonim district was always Belarusian. Modern Slonim district is  a modern day border between Brest and Grodno regions located in Grodno region.  Polish censuses of 1921 and 1931 were hoax acknowledged by Polish authorities. Poles needed to prove Polish majority in western Belarus, western Ukraine, eastern Lithuania to justify their occupation of the land.

PS Polish names and spellings of places in the land of Belarus are confusing.