How would you say "it is not easy" in Polish? One would use "lehký" in Czech for both "light" and "easy"...
no. while "ciężkie"-heavy can mean difficult/hard, lekkie rarely means easy. Easy is "łatwe". or "proste"- "straight"
Yes, in Russian it's the same usage as Czech. Eto nie liehko means "It's not easy," although легко liehko is an adverb meaning "easy" as opposed to легкий/легкая/легкое liohkij/liohkaja/liohkoje adjective meaning "light/not heavy".
Light means 2 things, not heavy and not dark.
True. But in Polish 'lekki' only means 'not heavy', although it can be more metaphorical.
'not dark' is 'jasny'. Which can also mean 'clear', like in "Yup, everything's clear".