Live browser Intl APIs — numbers, dates, relative time, lists, plurals, and locale-aware sorting. Change locale and watch formatting rules update instantly.
Decimal separators, grouping, currency symbol placement, percent, units, and compact notation vary by locale.
Date/time styles, calendar system, and time zone affect how the same instant is displayed.
Human-readable "2 days ago" / "in 3 hours" strings — grammar differs by locale.
Conjunction ("A, B, and C") and disjunction ("A, B, or C") with locale-specific separators.
Select the correct plural category — English has 2, Polish has 4, Arabic has 6.
Locale-aware sort vs naive .sort() — accents, case, and script order differ.