What rules of thumb govern when to pluralise a foreign word as it should be in the original language and when it should be pluralised as an English word?
For example, you'd get some funny looks using "octopodes" or "lemmata" in normal conversation. (I don't know what contexts would allow mention of cephalopods and mathematics, but you know...)
General rule of thumb is that words which obviously originate from Latin and are not in frequent use might be given the Latin plural (e.g. alumnus/alumni with 22m Google hits). More common Latin words usually have an English plural (e.g. campus/campuses 122m; bonus/bonuses 108m).
For languages other than Latin (and perhaps French), English plural is almost always used, even for rare words (e.g. klutz 5m), except where plural is the predominant or only form (e.g. spaghetti).
This is completely unscientific, my intuition as a native speaker (American). Counterexamples are welcome.
It doesn't seem to me there are any rules of thumb for how to pluralize a foreign word.
The New Oxford American Dictionary reports that both lemmata, and lemmas are the plural of lemma. If I check on the Corpus of Contemporary American English, and I search for lemma*, I notice that
Other examples of lemmae held to be doublets, and furnished with catchwords, but which actually occur three […].
If then I search for lemma* in the Corpus of Historical American English, I notice that the word lemmata is used for the first time in the 1900s; for the other years, before or after, the CoHA doesn't report the word lemmata.
For octopus the NOAD reports that
The standard English plural of octopus is octopuses. However, the word octopus comes from Greek, and the Greek plural form is octopodes. Modern usage of octopodes is so infrequent that many people mistakenly create the erroneous plural form octopi, formed according to rules for Latin plurals.
Looking for octop* in the CoCA, I found out that
If such a quality as intelligence can arise both in human beings and in the octopusan eight-armed sea animal without a bone in its body, then perhaps there is a course and […].
A search for octop* in the CoHA gives the following results:
Octopodes is used in one case.
In addition to being unspellable (octopuses, octopusses, octopoi, octopi, octopodes?), it conjured up every old horror cliche.
There are a few instances of words like octopoid, and octopodous.
Barney said, struggling free of the perfumed octopoid embrace, "just take your client aside for a moment then I'll be […]".
[…] Philharmonic Society, which, as I have already pointed out, is within the octopodous grip of the BBC even to the extent of having to satisfy that totalitarian body […].
What rules of thumb govern when to pluralised a foreign word as it should be in the original language and when it should be pluralised as an English word?
I don't think there are any rules at all. For example "spaghetti" is a plural in Italian but we use it as an uncountable noun in English.
Going through a list of Japanese words which have come into English (note that Japanese has no notion of plurals, uncountables, etc.), I see mostly cases where English has put whatever it finds convenient onto the word: "kimono" -> "kimonos", "satsuma" -> "satsumas", "miso"/"tofu" -> uncountable, as well as formations of nouns into adjectives, like "nashi" -> "nashi pears", "samurai" -> "samurai warriors", "shika" -> "sika deer", etc. Words which are specific to learners of the language, like "kanji", tend to be used in English with plurals without "s", e.g. "Remembering the Kanji" rather than "remembering the kanjis".
It looks like English is grafting these words onto itself in whatever way it finds most convenient, with the foreign plural being restricted to those who wish to demonstrate their knowledge of the foreign language ("spaghetti were" type of usages).
There is also the related phenomenon where we take a foreign plural and use it as a singular. Then we sometimes add an s to that for the/an English plural. I lived in Italy for several decades and whenever I returned to the U.S. it was always jarring to hear a café employee ask "Would you like another biscotti?" or ring me up with "So that's one espresso and two biscottis."
M-W defines the singular:
biscotto (n.)
plural biscotti
A crisp cookie or biscuit of Italian origin that is flavored usually with anise and filberts or almonds —usually used in plural m-w
I assume usually used in the plural means biscotti is OK as a singular.
The OED defines the singular headword biscotti and gives two plurals: "Plural unchanged, biscottis." It puts biscotto under Forms
For what's cooking, see this rather unambiguous Ngram
Collins Cobuild takes another approach. They define the plural noun but keep shtum about biscottis:
biscotti PLURAL NOUN
Word forms: singular biscotto or biscotti
Hard, plain, bar-shaped cookies containing almonds or hazelnuts Collins
Isn't that like defining dogs and telling you the singular form is dog? However, Collins seems to define its cookies, like macaroon (countable noun), in the plural. I guess you can't eat just one.
Summing up, the Italian biscotto/biscotti has become:
Singular | Plural
biscotto
biscotti biscotti
biscottis
Something similar happened with the Italian singular panino:
panini noun
variants: or less commonly panino
plural panini or paninisNOTE: Panini is a plural form in Italian but is commonly used as both a singular and a plural in English. Its use as a singular has given rise the variant plural form paninis. The use of panino as a singular form in English has become relatively uncommon as panini has become established as the usual singular. m-w
Sticking with Italy, there they call each noodle a lasagna and the dish lasagne (since they do serve you more than one noodle). We usually call both the pasta and the dish
lasagna
1 or less commonly lasagne : pasta in the form of broad often ruffled ribbons
2 a baked dish containing layers of boiled lasagna, and usually cheese, a seasoned sauce of tomatoes, and meat or vegetables m-w
The Italian brand, Barilla, has lasagne on their box of dry noodles. The Muller's brand seems to have an existential crisis. I see pictures with lasagna on some boxes and lasagne on others. In one picture, the instructions on the back of box for making lasanga include "16 oz. Muller's lasagne, cooked 7 minutes, drained"
I think Latin and French words are trickiest. The word cousin is French, but we have dropped the French variants for plural and gender, while we have retained it for fiancé and fiancée. Many native American words are spelled with French spelling conventions because French speakers used them before English speakers, but are pluralized as if they were English. Most words use Enlish rules, but some retain their variations such as alumni, alumnea, alumna.
