As you might read this:
http://sims.wikia.com/wiki/Songs_in_Simlish

There're some songs which only appear in UK English, French etc. but not in every game.
It depends on the language...

My game language is German, I found some special songs which only appear in German but not in English.

So is there a mod that unlocks all of the maxis songs in one language?

I mean an all-in-one package with all of the special Swedish, French, Dutch, UK English etc. songs unlocked for ex. US English or in my case unlocked for German?

You may want to go ask this at Simbology and see if they can help you.

Unfortunately, I can't create an account there...

PM me with your E-mail address that you use and I will get it to a mod over there.