That the pronouncing in Japanese is different, okay. But a R, that sounds like a D? However, in One Piece Animes, the name "Roronoa Zoro" is definitely pronouce with a R and not a D. I'm pretty sure, I would have noticed, if they named the guy "Dodonoa Zodo". I think, the speech recording here sucks.
Tahamh77 wrote this great explanation in a later exercise which answers your question really well:
「The complex-looking characters are called kanji. They are borrowed from Chinese, and are usually used for content words. Kanji is symbol writing, which means theres a different symbol for every word (although multiple kanji are often combined to form a single word, and the same kanji can have multiple meanings and pronunciations).
The simple, rounded script is called hiragana. It's mostly used for shorter words and particles like "wa" or "desu", although there are content words that use hiragana. Hiragana is a syllabary, which means that every character represents a syllable (except for the "n" symbol, which just represents "n"). In theory, all of Japanese can be written in hiragana, but this just isnt done.
Theres a third script called katakana that has all the same characters as hiragana, a bit like capital and lowercase letters in the Latin script. Some katakana look very similar to their hiragana counterparts, and some look nothing alike. Katakana is used less frequently than the other two scripts, usually just for loan words and onomatopoeia.」
I'll add to it by saying that hiragana is often written above kanji (especially in children's books) to explain how to pronounce them. This is called furigana.
Also, in general, numbers are written in roman numerals, although dates, advertisements, and prices on menus are sometimes written in kanji to give a 'fancy' or formal image.
Oh that's kind of lame; understandable, but lame.
Japanese people do commonly use Arabic numerals (123), so it's not wrong, but they still pronounce it as ろく. I would report it so that Duo makes people learn "six" = ろく or 六, otherwise we're not really learning Japanese ┐('～`;)┌
I just got 3 emails from Duolingo 10 minutes before you commented confirming that 六, 三, and 四, are now acceptable translations for their respective numbers. I'm not sure about the other numbers because those were the only ones I reported, but reporting them should hopefully result in them getting fixed soon. It seems like that was just an accident caused by an update or something, because I know for sure that kanji was accepted for all the numbers previously.