Song translation is a difficult art. Not only does the translator have to cope with rhyme patter, alliteration, metre, etc., but he/she also has to convey the meaning of the song and the allusions within it. Unless, of course, the song tune is just a vehicle for something completely different. All that's understood.

It can be done sympathetically, but is not necessarily a quick process.
Here's one done by the Mudcat collective over a period of 3 months!