Poor old Aristotle tried to logically quantify natural language. First-order logic didn't do the job very well. (But then, people don't even agree on his laws of thought.)
There are some factors which make natural language less restrictive than logical forms, and at the same time more confusing and less logical...
Natural languages depend very strongly on context in a way that is difficult to quantify; a statement will inherit context from previous statements even if the previous statement was from another person. Consider:
"How old are you little boy?"
"I am five."
On the most basic note, the sense of identity (that is, "am",) that the little boy uses does not relate to logical identity in any way. In the Propositional Logic, for instance, when you say P=Q, you mean that P is identical to Q.
The little boy doesn't mean that he is identical to 5, however. He means that he has the property of being five years old. But, since the "am" has a contextual ambiguity about it, this causes a real problem for the Propositional Logic. Look at the following sentences:
"Two plus three is five."
"Robert is five years old."
"A horse's head is the head of an animal."
That third one blows PL right out of the water, because it says that an object is a set of objects that it belongs to, and PL translates "is" to "is identical to".
Anyway...
When the boy says "I am five" he means "I am five years old", because he inherits the "I am X units old" from the person asking the question. The "years" is derived from social convention, a very wide context by which we expect people to measure their lives in years. The "How many units old are you" context, however, is borrowed immediately from the question and assumed easily by both speakers.
Things like this don't work very well with such logical languages as we have; even logical languages are Turing-undecidable, and they are not nearly as complicated as natural languages, which we have so far failed to quantify.
Most languages have a few words that other languages don't. This is VERY RARELY due to a lack of a fundamental concept... usually it has to do with a cultural specialization which resulted in a single word being invented to describe a complex situation, or a series of words being invented to differentiate a group of things.
For instance, the German language generally permits compound words in a manner that English generally does not; a "sehenswurdigkeit" is some thing or place that is worth seeing, a "windschirmwascherarm" is the arm of a windshield washer. Technically English doesn't have a single word for these, and must use two or three, but the communication is not diminished.
On the other hand, when we look at highly technical terminology, one language may have terms that another does not because they are not of concern... the Inuit words for different types of snow, for instance, don't really interest the rest of us because we don't have any technologies that depend on differentiating one type of snow from another.
Lastly, languages usually have vast differences of idiom, which are the most difficult things to communicate. When a German speaker speaks English using the German idiom, word order and so on, it generally doesn't sound too bad, just a little awkward -
"Wie geht es dir" is "How goes it to you"
German uses Time Manner Place for descriptions, so you would say "I am going on Friday by car to the drugstore."
However, this is probably because German and English are not so far removed from one another in the grand scheme of things. Translating idiomatic expressions from Japanese seems to be a real problem, if you ever get an anime disk and watch it with the English soundtrack and the subtitles on at the same time...