I am a professional translator with years of experience in both business and literary fields. The business side may pay the bills, but the song lyrics side is certainly more fun! It’s also a lot more complicated than you might think.
What a lot of people don't know about translation is that it requires much more than just knowledge of two languages. People may assume that the most “correct” method is to translate each word literally. The problem with this approach is that language isn't really made up of words, language is made up of thoughts. If you translate each word separately, you'll end up using words that are wrong for the context, and you'll miss the overarching meaning that the original author was trying to express. Your translation will not only sound unnatural, it will lack emotional power, and in my opinion, emotional power is the most important element in song lyrics or poetry of any kind.
My goal is to have the readers of my translations respond to the translations the same way readers of the original text respond to the original text. When I translate song lyrics, I try to render them in English that sounds to native English speakers the way the Japanese sounds to native Japanese speakers. I try to speak with the voice of the author. In order to do this, I might alter word order, add extra line breaks or reverse the order of lines. I might even add or delete words if I feel that it better expresses the tone and intention of the original writing. However, I never change meanings just because I feel like it. I aim for my translations to have vim and verve, but I try to respect the choices of the original author as much as possible, if I feel that they are conscious choices rather than obligatory conventions of the Japanese language.
A thorough knowledge of Japanese allows me the freedom to play with the language this way rather than just focusing on grammar. I know how words are functionally used in addition to what they mean. Too often, the word meaning written in the dictionary isn’t quite the right meaning for the context of the song…something all aspiring translators and people who are thinking of arguing with me about the accuracy of my work should keep in mind! Sometimes there is a right and a wrong answer.
Of course, there are also many places where the original writing is ambiguous, and in those places, I have done my best to preserve the ambiguity, or written my own interpretation of what the lyrics mean if the former is impossible. Japanese is a language second to none when it comes to ambiguous phrasing, and some poems could have five or ten different translations, each of them equally accurate. Japanese fans have lengthy discussions about lyrics meanings all the time! After a certain point, it's a matter of taste.