English is my second language. I used to love heavy-metal songs.
When I learned English, I realized what horrible things the songs were talking about. I was horrified.
I'm not the only one. I know some people who used to like hip-hop until they realized what the lyrics were talking about.

Many people around the world listen to American music without even knowing what the songs are talking about.

Do these songs have a negative effect on people who don't understad them?

Dr. Seuss books sometimes don't make much sense or aren't considered to be great writing, but the way that they flow makes them loved by both adults and children.
It's not the meaning of the words, but the sound of the words and how they flow that has an impact.
But then again, Chlamydia and Venereal have nice sounds too.

Debateable...... For a song to have a negative impact, an understanding of the words is necessary, however, I do know infants that were startled by a song contained in Walt Disneys Fantasia that had no words......

A new study has revealed that stroke vistims recover faster when songs are incorporated into their treatment.

I dont think that they do.
If you're just listening to the sound, it doesnt do anything to you.
like for example a cat. when a cat meows, you dont know what its saying. what if it was telling you over and over to go kill your brother, you wouldnt know it. and it wouldnt do anything to you because all you hear is "meow, meow, meow"

so i really dont think it does anything if you dont understand it.
i mean, listening to heavy-metal before you knew what the words were didnt do anything to you, right?

Only if the music makes you feel bad. If you don't understand what they're saying and you still like the sound of their voice and music i don't see that that could be a bad thing.

If you don't understand the meaning of the words then they lack influence in that way, but the tone in which they are sung or said in the song will have an influence.