https://github.com/P3pp3rF1y/SophisticatedBackpacks/blob/d4d6eafcc24ed41419cf3c417dd81e0dae33f657/src/main/resources/assets/sophisticatedbackpacks/lang/pl_pl.json#L93C1-L93C69

Instead of "Otwórz plecam" it should be "Otwórz plecak". "Plecam" doesn't mean anything. "Otwórz plecak" mean open backpack.

This apparently was reported twice. have this fixed in 1.21 dev so it will be correct from there on.