LONDON—El Salvador may be touting bitcoin’s use to help its citizens living abroad send funds back home, but the biggest remittance firms are cautious about offering cryptocurrency services. In a move that could be a harbinger of crypto becoming a more popular way to send money across borders, El Salvador on Wednesday became the first country to adopt bitcoin as a parallel legal tender. President Nayib Bukele played up bitcoin’s potential as a remittance currency for Salvadorans overseas. But despite the possible long-term risk to their business if such moves escalate, only a few of the traditional remittance companies who send the bulk of cross-border transfers are dabbling. Any efforts to get into crypto could be a double-edged sword, pushing down the fees that form the bedrock of their business. “For Western Union and some of the other remittance providers keep in mind that most of the volume in the …