When I still actively played and wanted to try new decks easily 10 of my cards were proxies. No point in spending €20 on a rare if it won’t make your deck better. The people I tested with did the same. We didn’t even bother printing them though. We just wrote the name down and the properties if we didn’t know them by heart.
And like others stated there’s many formats including ones that even prohibit rare cards.
And before WotC came out with a decent online way to play we also had an online client in which you could import all the cards ever, though I doubt it was superlegal.
Foreign language cards are entirely legal in official competition. I bought many of my dual colour lands on eBay from a German dude. Always passed deck inspection. Granted that’s literally over 20 years ago and I never placed higher than 17th in qualifiers, but I can’t imagine they changed that rule.