For languages, yes, overrides could be stored in a cookie, although they could also be stored in the URL (especially for documents intended to be downloaded). I

For languages, yes, overrides could be stored in a cookie, although they could also be stored in the URL (especially for documents intended to be downloaded). It could also support both. The reasons you mention are valid reasons to allow an override. (Still, if cookies are used for this or any other purpose, all cookies and their values should be documented, in case the user wishes to adjust them (or copy them) manually, and/or to understand what they are for.) Logout (and the duration that the login data is kept) is the responsibility of the user agent. Unfortunately, none that I know of provide the user any control over that.