Junior goes down to "Boca" (Boca Raton) and has some conversations with his girlfriend relating to the topic of "mouth."
"Mouth" is a major plot point in that episode, which is titled "Boca." (Using your mouth, and, running your mouth).
Tony makes jokes about this by singing "South of the Border," but this may also point to where people speak Spanish.
I am not aware of any Spanish slang involving the word "Boca," but I can tell you that the Spanish subtitles on HBO for "whistling in the wheatfield" and "Bushman of the Kalahari" translate to "going down to the river" and "diver without an air tank."
[EDIT] the full name Boca Raton is the Rat's Mouth. I left this out because Jimmy doesn't rat for a few more episodes, but someone pointed out that "Ratting someone out doesn't apply just to the cops. In this episode, Bobbi,Carmela, and Tony all rat out Junior for eating pussy, Meadow rats out Coach Hauser to her parents, and Sil rats out Hauser to Artie. Tony snitches the diddler out to someone who has the authority to arrest him. "