Why does Bitcoin uses a Script language?
Yes it does use a language called script. The transactions have both a locking and unlocking script and it is there to allow the specification of simple conditions for spending so that transaction validation is not based on a static pattern and allows the spending of money to be programmable. The locking script normally contains the public key hash while the unlocking script normally contains a digital signature. The scripting language also caters for more complex multisignature scripts. If you just limited the processing to a signature verification then it would not allow for richer functionality and multisignature functionality.