Former Rep. Beto O'Rourke was the first to respond to a question in Spanish, the most common language spoken in the US outside of English. During the debate O'Rourke, Booker, Castro, and a moderator all either responded to or asked questions in Spanish.
Latinos are a growing voter base, and according to Pew Research, "a record 29 million Latinos were eligible to vote in [the 2018] midterm elections, accounting for 12.8% of all eligible voters, a new high." In 2018, an estimated 11% of voters were Latino and according to exit polls they favored Democratic candidates. (Of course, it's important to note that not every Latino voter is a Spanish-speaker.)
Searches for 'English to Spanish translation' spiked +300% since the debate started#DemDebate
More data: https://t.co/I0WiP7r7bt
— GoogleTrends (@GoogleTrends) June 27, 2019