Limon means "lemon" in Spanish. While it's a common word, its use in this song is quite unique and intriguing.
The phrase "frances limon" (French lemon) is used to describe the way someone speaks. It evokes a sense of something exotic, perhaps a bit tart or refreshing, and definitely memorable, making it an enticing word to explore in the context of the song.
The title is a language that does not exist. Enanitos Verdes give their narrator someone whose voice tastes of lemon: "Cuando me hablas en frances limon" ("when you talk to me in lemon French"). Everything else in his world has gone soft and unreliable, starting with "Todo tiene el color de tus ojos, como ves" ("everything has the color of your eyes, as you see"), and that voice is the one thing holding still: "Todo cambia, no así tu voz" ("everything changes, but not your voice").
Then he promises to come back, and picks the least seaworthy vessel available: "En un barco de papel yo volvere" ("on a paper boat I will come back"). The city cooperates by switching itself off, "Las luces de la ciudad se apagaran" ("the city lights will go out"), and it all lands on "Te besare, me besaras" ("I will kiss you, you will kiss me"). The best line is the least romantic one, a small everyday blessing dropped into all that floating: "Me encanta tu actitud, Dios conserve tu salud" ("I love your attitude, may God keep you healthy"). The song opens on nothing but la-la-la and then forty seconds of guitar before a single real word arrives, which tells you how seriously to take the sailing.