So “läderlapp” is an old Swedish slang or colloquialism for bat. Back in the 50s when Batman was first brought to Sweden English literacy was a lot worse than it is now, and translating titles and names were much more common. A direct translation of Batman would be “fladdermusmannen” - which is a very unwieldy name - so they settled for a then-more-common slang for “the bat”. It does sound a little better, though it would have been best to just do what finally happened from the 80s and let Batman remain untranslated.
So “läderlapp” is an old Swedish slang or colloquialism for bat. Back in the 50s when Batman was first brought to Sweden English literacy was a lot worse than it is now, and translating titles and names were much more common. A direct translation of Batman would be “fladdermusmannen” - which is a very unwieldy name - so they settled for a then-more-common slang for “the bat”. It does sound a little better, though it would have been best to just do what finally happened from the 80s and let Batman remain untranslated.