In English the normal everyday speech is considered to be "plain English". So the phrase "Speak English" is immediately understood as a request to speak in simpler terms, similarly you can express your frustration at someone for not following simple instructions by "I've explained it to you in plain English". Well, yesterday reading Alice in Wonderland in Spanish I discovered that the phrase "Speak English!" (addressed to Dodo in the third chapter) is translated as "¡Habla en cristiano!" in two different translations of the book. Apparently, Spanish language considers the normal everyday speech to be a "Christian" speech.
In Russian one can either use the phrase "simple Russian": (I am telling it to you in simple Russian - Я тебе простым русским языком говорю) or "normal human language" (Can you explain it in normal human language? - Ты можешь объяснить нормальным человеческим языком?).
What about other languages? And what about Ladino, the language of Sephardi Jews, which is based on Spanish?