With the exception of (one) phenomenon, (many) phenomena, hopefully?
Also, now that I think of it: pegasus/pegasi, prolegomenon/ prolegomena, lemma/ lemmata, schema/ schemata, etc etc. There's one ending in -omenon that I keep forgetting...
For example, Samurai and Kimono are both loan words - but we import the pluralisation of Samurai (Seven Samurai) [1] whereas Kimono pluralises with an s on the end (Seven Kimonos) [2].
[1] https://www.merriam-webster.com/dictionary/samurai [2] https://www.merriam-webster.com/dictionary/kimono
This site lets you search for English words by suffix, and the only words it can find ending in "-omenon" are "phenomenon" and... "superphenomon". What a phenomenal result.
Though Australia is in the antipodes (to the UK), not the antipuses.
I don't know either percentage estimation or comparison with other languages, but significant portion of Russian common vocabulary comes from other languages. Major influences are Greek, Tatar, Turkish, latter, French, and, most recent, English.
About the same %, i think. The only difference is russian language can absorb almost anything, applying usual rules - so newly imported words are feeling native almost immediately
That's probably it: it's not the origin of English's vocabulary that makes it difficult, but its bad habit of not adapting borrowed words to conform to the English system.
If English speakers would rework spellings and plurals to match English when borrowing a word, it would be a lot easier for everyone almost all the time (the only exception I can think of is the rare situation (of questionable utility) where an English speaker can recognize a borrowed word in its foreign context).
How much of Russian's vocabulary is "native"? My understanding it that English's vocabulary is pretty messed up because it's a mongrel language: Germanic substrate with substantial portions replaced or augmented by medieval French, with lots of Greek/Latinate vocabulary for "educated" topics. That's made even worse because it tends to adopt words complete without translating foreign spelling systems (notable recent example: Pinyin). Things would be a lot simpler if the vocabulary had evolved from the original Anglo-Saxon substrate.