The film is Scorpion Spring from 1995.
According to this article from December 2016:
McConaughey followed up with a doozy of a story from his 1995 film
"I had this idea that I need to go back like I did in my first film,
where I just knew my man, and I would show up and just play the
circumstances - improvise,” he explained.
“So I said, “I’m not going to read the script. Just tell me the
character, tell me the situation, and I’ll show up, and I’ll just
react and do what I would do.
“So I show up on set, we’re about to do the scene, and I said, “You
know what? Since I know my man, let me just have a peek at these sides
real quick.” I pick it up: four-page monologue … in Spanish.”
It seems that Matthew McConaughey did not have an issue mentioning the film and the story in other interviews. Here is an excerpt from a 2013 interview with the Hollywood Reporter where he names the movie:
I go do this film, Scorpion Spring, and I got this idea: "I'm not
gonna look at anything. I know what I am: the drug lord on the Mexican
border in Texas. I'll just show up on the set, stay fresh and loose."
Well, I get down there, OK, and I pick up this scene, and it's a
page-and-a-half monologue in Spanish.