Can underskriver also be used in the meaning "to confirm", similar to the Dutch "onderschrijven"?
I do not think so, "underskrive" is simply sign, as in a contract, whereas 'confirm' is "bekræfte".
You could say something like "Kan du skrive under på det?", or "Han underskrev sin egen dødsdom", but I guess the meaning is somewhere between confirm and accept.
So there are different meanings of sign, would one say underskriver-sprog or is there another word for that?
No. "Underskrive" is a verb, and it can only mean to place your signature on a piece of paper. It literally translates as "underwrite" Then there's "sign", used as a noun ("it's a good sign"/"that's a sign of quality" etc.), which is "tegn" in Danish. So sign language is "tegnsprog". Finally, there's a sign on a shop or a highway, which is called "skilt" in Danish.