Locale
The setting that tells a program which language and regional conventions to use.
Typically written en_US.UTF-8: language, region, character encoding. It governs collation, decimal separator, date and currency formats, first day of the week and case conversion. Split into categories, so you can have English messages with local number formatting.
A classic trap: LC_COLLATE changes sort order, making shell scripts, database indexes and tests behave differently on different machines. Hence most build systems run with LC_ALL=C. CLDR is the modern data source behind all of it.