Undertale is known for its incredible soundtrack, which has apparently caught the attention of a TV producer in Spain who decided to use "Megalovania" in a reality show.
First Dates is a reality show that is filmed in a London restaurant, where strangers go on dates for the first time. The format of the show is simple enough that it has been adapted in several different languages, with versions of First Dates being broadcast in seventeen different countries outside of the UK.
It seems that a member of the production crew of the Spanish version of First Dates is an Undertale fan, as a Twitter user named aicontepig has uploaded a clip from an episode which used "Megalovania" on its soundtrack.
The song "Megalovania" can only be heard by taking the most violent route in Undertale, which involves slaying every monster in each area and doing as much harm as possible. Those who make it to the end of the game will have to fight against Sans in what is considered to be the most difficult battle in the game. "Megalovania" plays during the Sans battle and the song has become associated with difficult or epic boss fights.
It's unclear why the Spanish version of First Dates decided to use "Megalovania" in the show. It's possible that someone on the production wanted to liken the experience of going on a blind date with that of one of the most difficult boss battles in video game history. The more likely answer is that the producers had no idea of the song's origins, otherwise they wouldn't have used copyrighted music on the show
The use of "Megalovania" in First Dates is even more baffling because it doesn't match what is going on in the show. The sight of a couple of waitresses handing out shots doesn't require an epic video game boss song to accompany it, nor does a date between two people who have never met before. Undertale actually has a dating song, called "Dating Start!" that would have been more appropriate for the scene, but that must not have been epic enough for the First Dates crew.