I tried a lot of combinations but I can not convert it to English language. Although I wrote en-US like in function example, it doesnt work. I’m using Turkish LibreOffice, so text is Turkish.
=MONEYTEXT(D1;“USD”) it works, but =MONEYTEXT(D1;“usd”) doesnt work. When I try it from function windows, it also doesnt work.
I tried to change my LibreOffice language and tried the extension, it also doesnt work for language.
Haven’t the Numbertext extension currently installed myself, but the second argument likely is case significant, so for US-Dollar it must be upper case "USD", as you already found out. If you want the text in a different language you can try with a third argument, without such argument probably either the system UI or locale language is used. So, =MONEYTEXT(D1;"USD";"en") for English.
The extension’s pkg-desc/pkg-description.txt also mentions =NUMBERTEXT("help") to display some extra help. See also answer to this question.
… and the new [NatNum12] cell formatting option 
Yes, thank you. Maybe I tried “EN” with uppercase. I also tried en-US in the example they didnt work.