User locale is a property of ad_conn, ad_conn locale. The request processor sets
this by calling lang::conn::locale, which looks for the
following in order of precedence:
Use user preference for this package (stored in ad_locale_user_prefs)
Use system preference for the package (stored in apm_packages)
Use user's general preference (stored in user_preferences)
Use Browser header (Accept-Language HTTP header)
Use system locale (an APM parameter for acs_lang)
default to en_US
For ADP pages, message key lookup occurs in the templating
engine. For Tcl pages, message key lookup happens with the
_ function. In both cases, if
the requested locale is not found but a locale which is the default
for the language which matches your locale's language is found,
then that locale is offered instead.