The literal translation of nulidad de pleno derecho is "null act of full right" or more ideomatically "fully null" but the phrase of equivalent meaning in English language legal terminology is void ab initio, literally "invalid from the beginning".
This is an act that, because it was particularly seriously defective, should not produce any effect and, if it does, can be annulled at any time without the correction of the defect or the passage of time being opposed.
ex tunc means in a legal context, "from the beginning, from the outset. Used to describe certain legal effects that can affect situations prior to this point in time and therefore can affect past actions." It is more commonly used in canon law, Italian law and the law of other countries that speak Romance languages, than in the English speaking world.
You are correct that something that is nulidad de pleno derecho does cause effects ex tunc.
The phase ex tunc is rarely used in English terminology although the related term, nunc pro tunc which means having retroactive effect to some relevant date (often the date of the filing of a motion) is commonly used in cases where something is given effect retroactively, but not where something is invalidated retroactively, in which case the term void ab initio is used in English jurisprudence instead.
If an action is nulidad de pleno derecho then any legal effect it was given is treated as though it never happened and the remedy of restitution is often available to undo changes in position that occurred as a result of a void act.
Some of the common situations where this might come up are (1) an annulment of a marriage of someone who was not eligible to marry (for example, due to an existing marriage or lack of age to marry or mental incapacity) as opposed to a divorce, (2) a determination that a court order or actions are void because the court lacked subject matter jurisdiction to consider the matters before it, or (3) a determination that a deed to property and subsequent deeds in the chain of title derived from it are void because there was some serious defect in the original deed (e.g. it was a "wild deed" purporting to transfer say, the Brooklyn Bridge from someone who never owned it in the first place).