Info on profile page
I would really appreciate it if on a user's profile it displayed their native language, and also how long they have been a member of Duolingo, e.g. "Joined 5 months ago"
I think that it is up to the users what they would like to share, though it would be nice to know for yourself when you started learning.
Yeah, but at least it could be an option to have that information shown to others.
It is an option. If someone chooses to write the date they began their lessons, they're more than welcome to do so. This is more a tool for learning than social networking, though, so I don't necessarily see how valuable it would be to display such information to other users.
This is more cosmetic than necessary but it would still be a cool thing, although the profiles feel clunky enough already with a tree having to load and such. I think it wouldn't be an issue to add though, and it would add more personality to each profile (public or not).