I have the impression that it's true, since I know and have met quite a few young Finns who speaks some Japanese.. Some from studying there, having friends from there, interest in the culture, traveling there, some from watching anime.. But I wouldn't know if it's common overall..
I know multiple Finns who speak Japanese, but it mostly comes from anime fans or having other connections to Japanese culture from somewhere else. I wouldn't say many, but some. KuuttiSiit's comment about German, French and Spanish is more truth, but mainly because they are languages most schools teach as optional courses.
At least the Esperanto course here in Duolingo uses weird sentences like "Mi fartas bone, ĉar mi havas anason" (I feel well, because I have a duck"). I think the logic behind is, that such sentences teach the structures better by having some unexpected, cf. "…because I've been in good health". Perhaps they are using the same idea here.
Learner's Dictionary by Merriam-Webster (the official US English dictionary) has an explanation about the English usage.
Using that English approach moni and its synonym usea(*) can either be determiners or pronouns, see the Wiktionary article. I'm not sure about the style difference the article claims to exist, but the singular expressions are often perceived as stronger, more emphasising.
*: If you ask me, I would say, that there is a subtle nuance difference between those. I understand usea to mean a greater number, greater part of a group than moni, cf. many – several.
dieprinzessin is right. Don't forget that English has words that we tend to think of in relation to plural things or people that take a singular form after them: for example, we say everybody, not everybodies.
The "many a" structure is not particularly common, but it's perfectly good English. Perhaps one of the places we use it more is in proverb-like statements or if you want to give what you're saying a literary feel.
Why does this sentence use "puhuu" instead of "puhuvat"? Isn't "puhuvat" more appropriate since the subject (the many young finns who speak Japanese) is plural?
EDIT - I think I figured out why! The phrase "nuori suomalainen" by itself is in the singular (I.E "young finn"). So, while adding the "Moni" does change the meaning to "Many (young finns)," the phrase "nuori suomalainen" is still singular, thus the verb is singular as well (it this right?).
I would say that this sentence is comparable to the (somewhat old-fashioned) English sentence: "Many a young Finn speak Japanese" where the verb "speak" is indeed conjugated for the singular (b/c "young Finn" is singular) even though the "many a" lets you know I'm talking about many young Finns.
okay so i speak and live in finland, but im trying to improve my finnish a little, so thats why im using duolingo. but i think the "moni nuori suomalainen puhuu japania" is grammatically incorrect. the "suomalainen" is wrong, but i cant correct it, cause my finnish isnt the best. can somebody else maybe correct it for me? or am i just dumb and its actually "suomalainen"?
It is actually "suomalainen". There is a similar structure with singular in English, "many a young Finn", but for some reason the plural one, "many young Finns" has almost completely replaced that. In Finnish both are used with a nuance difference that the singular expression is considered stronger, more emphasising.
I would say that the singular version in English, when it is used at all, is also more emphatic. Perhaps this is simply because it is more unusual and thus sounds more poetic, as though the speaker is putting more effort into expressing the thought. "Many a time have I wondered ...." sounds like it is going to be followed by something more profound than "I have wondered many times ...."