Hey, I learned a new word today! Translyrics...
Speaking of which, I have some lyrics sitting around that have been like, 90% complete for around 5 or 6 months now. I should finish them sometime. Either way, I've only done this a few times (e.g. twice to any state of completion, among various other abandoned efforts), but I might as well share what I've found out myself. Maybe you've run into this stuff yourself, though.
Fair warning: I've been told I'm really, extremely picky about these, and I think I agree. I actually assume that I can't write anything I'll be happy with unless I have multiple bouts of inspiration that make me think otherwise. The songs weren't made with English in mind after all.
The big barometer test for me is singing what I've written. If at any point I think, "What the heck am I singing?" then that's a good sign I need to go back to the drawing board... in other words, I listen to that voice in my head that says I sound like an idiot. You should too. ^_^;
I don't feel the need to do a line-by-line translation, or even a literal one. Not even official translyrics do that when they exist, and honestly... literal translations aren't fun at all. So I don't do them. Heck, if the song has English lyrics already and they sound like random words out of a dictionary... I might get some hate from purists, but if there's no way they can make intelligible sense, they gotta go.
Sometimes if a line just isn't working out, I look at the rest of the verse and see if there's some reshuffling I can do. Sometimes I'll feel like I have to play with the notes as well to make things work. Every time I do that, I ask myself if it's really necessary, but sometimes it's okay as long as you can make it sound like it was part of the song originally.
Next important thing for me is figuring out how each phrase flows, and for me, it's not enough to make the syllable count match up. I'm not sure I can explain this very well, but more or less, there are some natural word boundaries in phases (which can change depending on the language), and it's awkward to keep a word going through them. Also, some phrases have a point where they're most accented, and some have a section that's not as prominent as the rest. When I see this happening, I try to fit the important parts in the sections that stand out, and leave the rest for the supporting ideas... if you understand what I'm saying.
There are also words that can stop the flow of the phrase, or start it/end it more abruptly than you'd like. Then there are those times when you realize the words you chose make the song into a tongue twister. Nothing you can do except find another way to line up your words.
If you find you've translated a section using way fewer syllables than was in the original song, you can try to get creative with what you have already. You could also find another way to say the exact same thing again, and repeat it for emphasis. You could also add embellishment... just whatever you think'll sound good in the end.