Allow non logged in users to change the displayed language
I noticed that a non logged in user (or a user without account) can not change the displayed interface language.
This can be annoying if your working in a multilingual team and you want to explain to a user how the system works. The user first has to create an account and go into his account settings to be able to change the language into something else. This should not need to be that complicated.
As a solution I suggest a simple dropdown box in the header or footer that displays all installed languages in that install and allows the user to pick one. This setting should be stored in a cookie so that the chosen language can be restored when that user returns. A lot of websites use this method and it should be easy to implement this into Redmine.