Improvements are made to Bitcoin's scripting language via Bitcoin Improvement Proposals, or "BIPs." Not all BIPs are focused on script, but some are. New op codes are introduced and agreed upon by developers and miners. Recent examples of this are BIP 65 and BIP 68. One of the benefits of BIP 144, or "Segregated Witness," is that its adoption should make scripting upgrades even easier.
It is important to note that a more sophisticated language alone may not solve all your smart contract problems. The other major hurdle is statefulness. Bitcoin natively supports two states: spent and unspent. Ethereum and Rootstock support a wider variety of states, which I believe Bitcoin would either have to hard fork or build via side chains.