- 20,000 is 20 k or 20 grand.
- 1900 is 19 hundreds not 1 thousand 9 hundred.
In Japanese numbers are also different:
- there's a word for 10,000 (man), therefore 20,000 is not "20 x 1,000" (ni jyu sen) but rather "2 x 10,000" (ni man).
- there's a word for 10^8 (oku), making 1 billion = jyu oku (10 * 10^8).
The lower powers 10–1000 take a prefix of 2–9 to multiply them, and omitting the prefix means one.
1 ichi
2 ni
10 juu (not ichi juu)
20 ni juu, literally two ten
100 hyaku
200 ni hyaku
The powers of 10000 seem to take an obligatory prefix 1–9999, formed the usual way (I've never heard just "man" for 10k, always "ichi man", and 12340000 would be literally translated to "thousand two hundred three ten four ten-thousand", while none of the smaller powers seem to ever take a prefix > 9.)
The above is based on not living in Japan, though, so the sample of numbers I've seen might well be missing some things that occur in actual native speech by someone not teaching a first-year class.
Largest one I've seen is 10^68.