As a teacher, I found that theory is by far the easier of the two to teach. Theory, unlike language, follows a specific set of rules. And once you understand the vertical and linear relationships, you can cognitively understand how to put it together.
Language on the other hand is a bit more complex. One, it's not as easily codified. There are many languages which have evolved from the many styles of jazz, and there many interpreters who have created these different languages. In addition to the innumerable note to rhythm combinations, language can only mastered through the experience gained from years of playing, imitating and listening to others who have mastered the particular vernacular you're trying to perform.
As a jazz student, it's important to remember that just because you got an "A" in Jazz Improvisation 101, you could still, however, lack the performance skills needed to be an effective improviser. It it for this reason that I always stress that you take the necessary steps to complete the other half of this puzzle--which is learning the language on which the theory is based and honing that language through playing.
What you can learn from a book in the classroom has it's limitations, whereas, the possibilities of what can be learned on the bandstand are limitless.