I think the answer would be
I'm understood by pairs of trained eyes
words are only understood by us, humans, that have enough brainpower to make sense of letters put together - words.
For all the rest I'm nothing but lines
words span across multiple lines. Just like they did in this answer.
I can be copied, and still I'll be me
If you copy it you must also attribute copyright ownership though ;)
I can live on paper, on tongues, perhaps a CD
People speak and write words. Installation CDs, in particular, usually have some text written on them.
How would you feel if I referred to you as 'humanity'?
I am only able to answer that question, because I can make sense of "humanity" is. Otherwise humanity would be just a bunch of letters. This is very similar to "I'm understood by pairs of trained eyes"
Whom does the poem describe?
Needs to be a thing or a person. In this case, words!
Was four, am six, I'm very concrete
This verse may be a bit trickier. I think it's trying to describe that words usually are different from each other when put together to form a sentence. Hence (I) was four (the word four was just written), (I) am six (as just written).
You're probably staring at the thing I depict
I am indeed!