Author here. I'm working on launching a new webapp dedicated to helping language learners master numbers both in production and listening comprehension. I'd love any feedback, tips, or suggestions from the HN community.
On the main page, consider listing the languages, rather than using flags. Languages and countries don't always correspond one-to-one; many of the flags you have link to the same language, and conversely there are multiple languages spoken in some countries. A language list would be both more compact and more directly convey what people want to look for, rather than making people indirect through "I want to learn more of language X, which of the several countries speaking that language should I select, and does it make a difference?".
(That said, if you later expand to cover regional differences in accent, distinguishing between countries seems useful for that.)
As a side note, 'Finnish' is misspelled.
Suggestion: make the lowest time at least 20 or 30 seconds, as 10 is too short.
Second suggestion: a visual component where you could utilize shop signs with prices on them (http://i.imgur.com/wsg5BU4.jpg) but I'm not sure how that could benefit the user, maybe the user has to type out the number (like "fifty seven").
In Chinese, the number "100,000" is not read as "100 thousand", rather it is "10 ten-thousands". This makes translating numbers in the range of 100,000-10,000,000 a bit awkward if you are not used to it.
"72" might be "seventy-two" but it is also "two plus seventy."