frajer in Czech can have both negative and positive meaning, depending on the context, but usually we say it as a positive word - often meaning a "cool guy" or something similar. It can also be a general word for a guy or for a boyfriend
I see Polish has it quite differently
Not such a widespreaden false friend (just one case), but once a shop had for their catalog the following thoughts "Yeah... Rucksack is such a lame german name for this item, we need something cool in english!" and they called their rucksacks (which is an existing germanism in english for a special type of backpacks...) "body bags" which means Leichensäcke in german.