Hmm. I looked that one up, because I figured I'd make that mistake. Il est is used when referring to something already mentioned. Since I mentioned l'allemand, I figured that would be the correct "It is".
Your logic is not wrong. Instead of repeating "L'allemand" (Est-ce que vous aimez l'allemand? L'allemand est une langue que je veux apprendre.) which would be correct but not very natural, you used a pronoun. You just did not pick the right one, but I would have a hard time explaining why, so I'll just throw some examples at you:
Bob, il est grand. (adjective)
Bob, il est m?decin. (profession/religion/nationality can be used like adjectives to describe someone)
Bob, c'est un m?decin. (noun - with determiner - the meaning is identical to the previous example)
Bob, c'est un ami de ma soeur. (same structure as above)
L'allemand, c'est une langue que je veux apprendre. (same structure as above)
NB: Bob, c'est grand. (A bit strange, but could be correct if Bob did something great and the sentence was the title of a newspaper article or something similar)
Now just to mess with our heads:
Le fran?ais, c'est difficile. (adjective... and now both our heads hurt)
Le fran?ais, il est difficile. (the sentence is correct if "Le fran?ais" is to describe a specific french person or an archetype)
La langue fran?aise, elle est difficile. (heads still hurting)
La langue fran?aise, c'est difficile. (Also correct. Heads are not feeling any better!)
Mais bonne chance!
