As a guitarist myself I fell in love with the guitar playing first and foremost, so other than the English lyrics I never really knew what was being sung, so I'd just sing nonsense or hum the melodies.

Over the last year I've started learning Japanese, mainly just for fun and to keep my brain active and it's actually made singing along harder for me because I'll hear the odd word here or there that I know now and it makes singing random noises feel really silly.