It doesn't sound like English, at least in my brain it doesn't register as English, and I may have found a plausible reason: If you look up the amount of English words with the letters "iarda" on them you only find 3 words (at least according to wordmine.info), but French shows 38 words, even just "arda" the difference is huge: 317 vs 2017 words https://www.wordmine.info/french/words-with?minletters=2&max...
Well, it's mostly the -aire ending that is uncommon in English, because it comes from French, as far as I know all -aire words in English are straight French loanwords. It's somewhat equivalent to the English suffix -ar.
That makes millionaire and billionaire about as unenglish as milliardaire, I think it just doesn't sound weird to you because you see them more often.
