In Linux, locale
definitions (provided by glibc and glibc-i18n packages in Slackware), are customarily stored in /usr/lib/locale.
In /usr/share/locale you will find mostly <locale>/LC_MESSAGES subdirectories, that host files ending in .mo. These files are translations of application messages in the given locale, in the Machine Object (MO) format defined by gettext here
. /usr/share/locale is the default location for these files.
See also Internationalization variables
in the POSIX specification and Locale Environment Variables
in gettext's manual.
Long story short: to translate messages of an application in a given language you need a proper MO file in /usr/share/locale/<locale>/LC_MESSAGES *and* locale definitions in /usr/lib/locale.