Theres probably no harm in allowing it to work with reading, I could see it being read either way. Comprehend language is a first level spell that lets you understand and read anyway.
I think it is intended to include reading. It doesn't make sense to me for a spell that appears to be an upgrade on a lower level spell.
On the other hand speaking in tongues has not always meant reading and speaking multiple languages. In the Bible when the disciples spoke in tongues it was described as people across ethnicitys and languages hearing them in their own tongue.
It only allows speaking, AFAIK. Comprehend Languages specifically mentions reading, while Tongues doesn't. I agree that letting it include reading makes sense, and won't hurt anything. You expect a 3rd level spell to do a lot more than a 1st.