During the last years, blockchain and smart contracts are receiving substantial mainstream attention from academia and industry. Blockchain is a distributed database that can be seen as a ledger that records all transactions that have ever been executed. In this context, smart contracts are pieces of software used to facilitate, verify, and enforce the negotiation of a transaction on a blockchain platform. This study aims to (1) identify and categorise the state-of-the-art related to smart contract languages, in terms of the existing languages and their main features, and (2) identify new research opportunities. As a result of the review protocol, 4,119 papers were gathered, and 109 of them were selected for extraction. The contributions are twofold: (1) 101 different smart contract languages have been identified and classified according to a variety of criteria; (2) a discussion on the findings and their implications for future research have been outlined. At first, almost 36% of the studies have been collected from grey literature (GitHub, webpages, etc.), which proves the importance of the industry in this field. We have identified the current challenges to reveal several gaps and raise open problems within the field. The most interesting open problems include the need to improve the developer coding experience by providing tools to write smart contracts as human-readable as possible, and the need to deal with Oracles and trusted off-chain information and processes. We find out many smart contract languages and blockchain platforms, making the ecosystem rather chaotic. However, there is no common or standard language to specify smart contracts that are valid regardless of the blockchain platform. This multivocal mapping study provides a snapshot of the smart contract languages field that serves as a baseline and a tool for future work, e.g., future surveys or future literature reviews in which particular issues or aspects such as security and privacy might be studied in further detail. To do that, we provide a bundle with all the resources at http://www.idea.us.es/smart-contract-languages/.