Spanish translation
for plea bargain

According to a Spanish dictionary I came across, here is a way you could translate plea bargain into Spanish:

Negociación extrajudicial entre el abogado y el fiscal por la que el acusado acepta su culpabilidad en cierto grado a cambio de no ser juzgado por un delito más grave

Now, the one thing that you'll notice is that the above translation isn't a succint two-word phrase like it is in English. This happens a lot, especially with legal terminology. This is because the legal codes are different in English-speaking countries than in Spanish-speaking countries and so many times short phrases above like plea bargain have to be explained in the translation.