OT: Diese Exhumierung war jetzt aber dringend nötig. Es ist immer gut, die Informationen eines längst umfänglich beantworteten Fadens alle 15 Jahre wiederzukäuen. Vor allem, wenn sich die OP-Frage heute über den "i"-Knopf des Wörterbucheintrags leicht klären lässt: https://www.merriam-webster.com/dictionary/diploma bzw. Ausführliches zur Etymologie. (Und den Nick von #6 finde ich weiterhin 💩.)

Der korrekte Plural für das griechische Nomen wäre „Diplomata“.

No. The correct plural for the Greek noun would have to be written using the Greek alphabet.

Since "diploma" is truly an English word (with Greek roots), its correct plural is "diplomas."

(And even as an NES, I agree with the objections to the the nickname in #6.)

As an aside, and for what it’s worth, the L. pl. of the n. (not f.) noun diploma is diplomata (not diplomae) – “diplomata” has also been used in the past as the pl. of E. “diploma,” and for all I know there may be one or two who are still inclined to do so. Both OED and M-W list “diplomata” as a variant. When might it be used? Maybe in certain historical works?

That said, it can’t be stressed enough that “diplomas” is the absolute standard plural. Don’t so much as think of using anything else.

(+1 re ##7, 8, 9 and the nickname)

As I read the M-W entry, the use of "diplomata" is restricted to a particular usage of diploma that is not the same as that related to a university degree, which is the subject of the OP (only #1, not #2 and #3, meaning that it's an alternative form ("also", which is less frequent than "or") of a single variant (which I don't find to be all that common anyway).

plural diplomas

1

plural also diplomata

  : an official or state document : charter 

2

 : a writing usually under seal conferring some honor or privilege

3

 : a document bearing record of graduation from or of a degree conferred by an educational institution

The OED on my Kindle (which is not the unabridged OED) says nothing about diplomata, but it does indicate that M-W's meaning #1 is historical ("an official document or charter" in OED)

As I read the M-W entry, the use of "diplomata" is restricted to a particular usage of diploma

Thanks for underscoring that about M-W as well as for drawing attention to the distinct senses of “diploma.”

The OED on my Kindle (which is not the unabridged OED) says nothing about diplomata

The online OED lists what I referred to as a “variant” thus: “Variant forms : Plural diplomas, sometimes diplomata”

OED differs from M-W in its division of senses as also in the senses to which it ascribes the historical pl. form “diplomata” (which it only does via its quotations, not in its glosses). The main senses there, and the occurrence of “diplomata” in quotations, as follows:

1.a.

A state paper, an official document; a charter.

[no instances of “diplomata” in 5 quotations 1645–1877]

1.b.

An original document as a matter of historical investigation or literary study; plural historical or literary muniments.

[3 instances of “diplomata” in 3 quotations 1845–1891]

2.a.

A document granted by a competent authority conferring some honour, privilege, or licence; esp. that given by a university or college, testifying to a degree taken by a person, and conferring upon him the rights and privileges of such degree, as to teach, practise medicine, or the like.

This morning our Diplomata were presented to us … to certifie our having visited all the holy places. H. Maundrell, Journey Aleppo to Jerusalem (1703) 109

[1 instance of “diplomata” in 10 quotations 1658–1863, dated there 1701, and the pl. “diplomas” not quoted before 1849]

@Rominara: bitte erst mal lesen!!

Das "denke ich" und ähnliche Einschränkungen sind nur auf die Vermutung bezogen, wie der Plural im lateinischen gebildet worden wäre (was aber für die Ausgangsfrage eh irrelevant ist), dass der englische Plural als "diplomas" gebildet wird, ist klar und deutlich genannt worden.

Grammatiknazi (ein klarer Nick nur um Aufmerksamkeit zu erregen, ich fürchte die Aufregung darum adelt ihn nur, ich schlage vor, ihn klammheimlich seitens LEO in seine Mitgliedsnummer ändern, sein login sollte davon ja nicht betroffen sein) hat hier nichts Neues gebracht. Er/sie hat auch nur den englischen Plural bestätigt, und ebenso bloße Vermutungen über den griechischen und lateinischen Plural angestellt. Eine Kritik an der Exhumierung ist also durchaus berechtigt.

Übrigens, wenn ich meinem Lateinlehrer von damals in der Schule glauben darf, haben die alten Römer die Begriffe aus dem griechischen auch in deren Ursprungsplural verwendet. Das Überstülpen eigener Grammatik auf offensichtlich fremdsprachliche Begriffe sei ein Unsegen der neuzeitlichen Sprachadaption. Also hätten die alten Römer in dem Bewusstsein, dass es sich um einen griechischen Begriff handelt, "diplomata" gesagt, statt eine lateinische Endung diplomae zu bilden. Ob das auch auf den "Römer auf der Straße" zugetroffen hat ist wegen bloßer schriftlicher Überlieferungen durch Gelehrte schlecht zu sagen.

A final footnote on the subject of "diplomata." In contemporary scholarly works, judging by the few examples I’ve looked at, “diplomata” appears to be being used either in OED’s sense 1b, and / or possibly 1a. To give just two examples:

Most of the diplomata are confirmations of immunities for ecclesiastical institutions, but there are also significant grants of lands to ecclesiasts, institutions, and fideles. The production of diplomata attests to particular aspects ...

(Laura Wangerin, Kingship and Justice in the Ottonian Empire (2019) – p. 44)

The diplomata are a particularly important record in Germany and Italy in the absence of wills with their records of gift giving. In contrast, the will of the ætheling Æthelstan (d. 1014) contains a wealth of detail about Æthelstan's ...

(Theresa Earenfight (ed.), Royal and Elite Households in Medieval and Early Modern Europe (2018) – p. 57)

In both these works “diploma” and “diplomas” are also used – whether always in the one sense or not I leave to others to investigate and decide.