Tor operates only on the IP layer, application layer data is passed unmodified. HTTP headers belong to application layer. It is not in the scope of tor to do "deep packet inspection" and mangle data of higher layers, and the tor program does not have functionality for that. And the exit node does not run an own HTTP client.
It is your browser's responsibility to send headers from a large anonymity set, to keep you indistinguishable from many other internet users. If you set your browsers content language to english, and user agent to something often used, you should be fine.
Firefox addon for spoofing user agent:
UAControl
Test how unique you appear:
Panopticlick. The lower the number, the better.
Or you could run a local proxy which mangles HTTP headers.