I've been reading a few volumes of manga, but because my level is still quite low, I usually follow along with english translations of the manga.

Often, the Japanese sentences will have a question mark at the end, and I can't quite understand why. Then, I check the english version, and it was just translated as a normal statement--not a question.

This is not just a few isolated instances, but rather, I see these sentences all over the place.
Does the usage of the question mark differ slightly in Japanese than in English?

Remeber that japanese and english are very much different. So like, not everything in japanese can always be translated into english. So they usually just get the "meaning" of the whole sentence and then translate it to english. So like for example reading manga, sometimes trying to understand it in english while reading something in japanese can be confusing at times. Like for me personally i was reading a chapter from the bleach manga, but that exact part in the english translation was something completey different. It's kinda weird to say just understand it in japanese, by itself. So i'd say try to decode what it is in japanese, i.e. searching in dictionaries etc until you get a general meaning or understanding of it.

AFAIK the usage of the question mark doesn't really differ, but can you give a specific example?

Looking over some of the sentences again, I think I can understand what's going on now.
For instance:
このままだと本当に命が危険なんだよ？
I'm guessing in this instance, the question mark itself is acting kind of like a "you know?" or a "right?"

Well, just remember that in spoken Japanese there is no strict rule on using か or any number of other things that would mark a question (though in that sentence the なんだ would kind of hint it's a question). So the question mark would most likely point out that it should be read in your head in a rising intonation and treated as a question, just like in English.

Oh, I thought this なんだ　is just an explanatory tone statement.

Yeah, it can be. But if it's used with an rising intonation that sounds like a question, it marks a question. The ん there is just a shortened の、and you can really shorten the end of the sentence to 危険なの？

That's my guess at least.

Nah, in this case it's definitely a statement, there's no way だよ could be used in an actual question, since both are assertive. What the question-tone of this sentence is aiming for is slightly different but similar to ね.

A fitting translation would be:
If it continues like this, there will be true danger for your life, (don't you understand that?).

That last part is of course only the nuance.

And I don't actually think なんだ can form a question like ryuujin is saying. Just feel these two sentences:
そうなの? - Is it so?
あ、そうなんだ... - Ah, so that's how it is...

I can't think of any example where なんだ sounds like an actual question.

I think it's because the intonation on よ rises, and it's pretty much the same as the intonation on か. 先生ですか。 and 先生ですよ。 have the same sentence-ending intonation, so they're using the ? to mark the sound of the intonation, not necessarily the presence or absence of a question. I don't think there's any way that something ending with よ can be a question. If there's no よ, then the question mark can be used to show that something is a question that might otherwise be interpreted as a statement, but here that's not the case.;

And you can use んだ to make a question (not with よ, though). 君なんだ？ is a more blunt version of 君なの?

Really, you think so? I think in spoken Japanese I've heard よ thrown around in questions occasionally. It's a little rare, but I've definitely heard it.

Though oddly enough, I've never heard ぞ or ぜ, but I guess that makes sense. They are a bit stronger.

どうなんだよ is a question

I've seen だよ as a question ender all the time. Its just rather rude and not really a question, just kind of a rhetorical remark or "who the hell are you" for example.

Erubey Wrote:どうなんだよ is a question

I've seen だよ as a question ender all the time. Its just rather rude and not really a question, just kind of a rhetorical remark or "who the hell are you" for example.

That's not the same thing though. That's actually when you're just asserting a question word, such as 何だよ!?. It's translated into a question in English, but from a Japanese standpoint, it really isn't. I guess that depends on your perspective though.

The only real exception I know of is the "かよ" ending which are real questions with よ at the end, but it feels more like an idiomatic usage of those two particles together.