If it's just a song that's nice to listen to that doesn't have any more meaning to me, then I'll just listen to it without bothering to know what they're saying. I know a few words here and there, but other than that it's mainly how it sounds than the lyrics themselves. On the other hand, for those songs that do have more meaning to be (it's a song from one of my favorite animes, etc etc) then I'll generally look up the lyrics so I could understand what is being said. One example would be the song in my signature. It's a fan song, but it's a song based on a poem in the manga, and knowing the lyrics to the song and understanding it just gives it so much more meaning.