Music is for there for people to enjoy, regardless of language or culture. Why must the question be raised if they understand, when it's really of no matter? I guess if you had a 10 year old who knew the lyrics exactly and the message behind the lyrics were very dark/sexual/etc, then that could raise issues if he/she went abroad etc. But since it's a foreign language to British people, they don't know what it means, or probably can't even say it properly. But I have no idea how many of those people would translate it through the internet.
I love listening to world music, whether it be eurotrance stuff (that Basshunter guy from Sweden is awesome), from Middle-Eastern and Asian music (I love Mayada Al-Hennawy, Lata Mangeshkar, Aitha Bin Tanaf, etc), to J-Pop (Like Utada) and more, and I think the whole concept of keeping it in the other language is one of the best aspects of it. It takes away half of the beauty of the song having it subtitled or translated for you.