If a user goes to the validator's home page, get a certain language (e.g en) with language negotiation, then chooses another language (e.g fr) with the language selection menu, it would be nicer that the validation results be en fr (the user's choice) not en (negotiated)

Simple and cheap way to do this would be to add 
<input type="hidden" name="lang" value="XX" /> into validator.html.*

(suggested by Jean-Guilhem. I agree)