I think that if the speaker knows the correct plural form of a word borrowed from a foreign language (including Latin), then the speaker should use the correct plural form of the word, disregarding the rules of English grammar. If the singular form of a foreign word can become part of the English language, then why can't the correct plural of the same foreign word become part of the English language also? Why do we have to continue attaching an "s" to every foreign word when the plural of that word already exists?

I give you two examples of Italian words that are now commonly used in English. "Innuendi" is the correct plural of the word "innuendo." Please don't say "innuendos." "Cannoli" is the correct plural of the word "cannolo." Saying "cannolis" is redundant.