If you think translating text is hard, imagine the extra work that comes with translating poems, songs, and musicals! You not only have to present the original text accurately, but now you have to keep the rhythm and rhyme scheme! I can’t imagine how one would even go about doing this, but luckily, Baptiste Deval explains step by step on Quora. The original can be found here: http://www.quora.com/When-a-musical-is-translated-how-do-they-manage-to-make-it-still-rhyme

Bringing a musical to a new country and a new language imply to do more than a translation: it is a work of adaptation, where rhyme is not the only challenge. You want to be able to translate cultural references that might not speak to your local audience, you want to respect the rhythm that was created by the original lyricist and composer, and of course, you want to respect the meaning of the original song!
However, you don’t need to stick to the exact same metaphors or vocabulary as the original lyrics: what you want is to convey an idea, a feeling etc… and most of the time, you need to get away from the original sentence in order to translate it correctly.

Typically, when translating a new song, I start with a rough translation of the lyrics. I look for the idioms and words that I don’t know, and quickly write a bad first version of the text (in French, in my case) to help me keep track of the general meaning of the song.
Then, I proceed sentence by sentence, letting words come to me. Sometimes, you will be surprise to see how good a basic translation works for one particular sentence, or that the sounds are true to the original rhyme.
Then, I explore the lexical fields, look for synonyms, and write all of my ideas: it’s not because I found a great translation to one sentence that I should stop here and not look for alternatives. I usually build a section around one or two sentences that I really like, but sometimes it simply cannot work and I would find a compromise.
Wordplays, metaphors and idioms can be the hardest to translate, as you might not have an interesting equivalent in your local language.
However, following this process, I usually find myself having almost 50% of the translation. And that’s where the hard work start.
At this point, finding solutions to the challenges I’m confronted with takes more efforts: I will come to the point where I sum up each stanza to its general idea and the way the author(s) have constructed the sentence. From this point, I will start finding all new words to give the same impression to the audience. You need to try (and miss) a lot until the point when you sorted out everything and you have full song!
You’re not quite done yet though: rehearsals and discussions with the director and the actors will lead you to adjust some of the lyrics before you have your dream translation!

As you can imagine, this is a lot of work, and success is not guaranteed (it really depends on the quality of the job that the translator did).