- All languages are interesting.
- Every language is interesting.
Don't they mean the same thing?
All and every are universal quantifiers. Two interpretations are possible, distributive and joint:
1. All of the students lifted a piano onto the stage.
All allows distributive and joint interpretations:
The distributive interpretation in this example is extremely unlikely. We would naturally understand example #1 with a joint interpretation, and we probably wouldn't even consider the alternative.
But every is a distributive quantifier and forces a distributive interpretation, as unlikely as it is:
2. Every student lifted a piano onto the stage.
We can add one after another to this sentence, but not together. Because every forces a distributive interpretation, this sentence is rather unlikely.
In your example, the basic meaning is the same either way, because it doesn't matter much whether we're thinking about the languages together or individually:
3. All languages are interesting.
4. Every language is interesting.
But every requires distributive semantics. In example #4, we think of the predicate ("is interesting") as applying to each language separately. All does not require this, and example #3 can be thought of as applying to all languages taken as a whole.
Examples #1 and #2 are modified from The Cambridge Grammar of the English Language, p.362. Note that each is more strongly distributive than every; for a detailed discussion, see chapter 4 of Susanne Lynn Tunstall's 1998 dissertation, The Interpretation of Quantifiers: Semantics & Processing.
Regarding the issue whether and which languages are interesting, both sentences say the same thing in my opinion. The difference is made only by the choice of "all" - a more general expression, possibly shifting the focus slightly on the fact that all languages have something in common, e.g. being a means of verbal communication - versus "every", which focuses more on the single items of a whole/a group.
And subjectively ... When I hear "All..." , I think there is room for another or an additional point to the joy of studying/speaking each particular one, which would be my impression when I hear the second phrase. "All" would probably come to my mind quicker, but I'd imagine it would be useful talking to someone who doesn't share my enthusiasm. It just conveys the idea. Using "every" makes this enthusiasm live a little more in the conversation :)
The first statement is called a Logic Tautology. It states a definition. Please see http://www.britannica.com/EBchecked/topic/584431/tautology. You may "define" that All languages are interesting. Or, languages = interesting as a formula.
In some sense it is an attempt to state a universal truth.
"Every language is interesting" implies a personal opinion, based on personal experience or beliefs. It is an answer to a question such as "Do you think languages are interesting?"
No, they do not mean the same thing.
When you say:
All languages are interesting.
all is really a generalization. All the languages you know are interesting, but you might not know all of them.
all could also be understood as most in this context.
Every language is interesting.
every means each language that exists, whether you know about it or not.
Also, every would be used when you want to emphasize each and every language.
The two sentences can mean the same thing, or they could mean different things. It depends on the context. Instead of languages, though, I'm going to use candles. You'll see why in a minute.
Consider the sentence
Every candle is interesting.
This sentence clearly means that if you were to individually investigate every candle, you would find it to be interesting. So in the first sentence above, it's like going around a house and looking at each candle and finding them all to be interesting. With that in mind, consider the following sentences:
All candles in the house are interesting.
All candles are interesting.
So far, we're still basically on the same page.
Now for a moment, consider that while each candle might be interesting on its own, its interesting-value might depend on the candles around it. So while in the above examples we were going around the house, perhaps we found almost all of them to be interesting, but a couple to not be interesting. However, once we put all the candles from around the house on the table together, suddenly even the non-interesting candles become part of a very interesting candle collection.
All candles are interesting.
Now, to be taken in this context, this sentence desperately begs us to put the word the in it, i.e. all the candles are interesting, or perhaps all these candles are interesting. If there was to be a candidate meaning to make those two sentences be different, this would be it. And it may be technically correct: in this case, "all candles" is a single entity. Yet, because it appears pluralized, we still use "are" instead of "is". However, I think even if it is technically correct (which I'm not convinced it is, but may be) it would still confuse the beejeepers out of an English speaker and should be avoided.
I can't think of any other possible way the two sentences could mean different things, despite the drive-by comment to your question.
No. I don't think they mean "exactly" the same.
All languages are interesting. -All languages that are there as of now are interesting. May be in future there will be a new language which is not.
Every language is interesting. - Every language is interesting. (generalized).
Had it been Every one of these languages , then it would have been the same.
You generally wouldn't say
All languages are interesting, instead you would say
All of the languages are interesting.
All is used to refer to sets or collectives, and not multiple individual objects, consider this:
All of the languages are interesting and
Every language is interesting mean different things. The first can only mean the languages you have mentioned or previously considered (the set) are interesting, and don't refer to a language that is not apart of the subject.
Every language implies every language, period.
All = 100% of 1 thing.
Every = 100% of all things.