Radio 4's Today programme this morning included an interview with Gibralterian historian Tito Vallejo Smith, who spoke about the local vernacular, Llanito, pronounced janito (listen again here - scroll through to 1 hr 57 min). The official language of Gibraltar is English, but just about everyone also speaks Spanish, and Gibralterians are very likely to change language mid-sentence when they are speaking.
Llanito is a mixture of Spanish and English, with borrowings from French, Portuguese and other languages. According to Wikipedia, the origin of the word is uncertain. In Spanish llanito means 'little flatland' (the land surrounding the Rock consists of plain and marshland). Another suggested origin is that Llanito comes from the diminutive of the Italian given name Gianni, given that in the late 18th century and early 19th century the majority of the male population of Gibraltar came from Genoa.
Tito Vallejo Smith has written a dictionary of Llanito. Here's an article in which he talks about how the language is currently under threat.
Apologies for my long silence. I've been having a very lazy summer!