Use when working with multibyte strings, UTF-8 encoding, character encoding conversion, locale-aware formatting (dates, numbers, currencies), transliteration, pluralization, or internationalization. Covers mb_strlen, mb_substr, mb_strtolower, mb_strtoupper, mb_detect_encoding, mb_convert_encoding, iconv, grapheme functions, emoji handling, unicode processing, locale-aware sorting, ICU library, intl extension (Collator, NumberFormatter, IntlDateFormatter, MessageFormatter, Transliterator, Normalizer), currency formatting, date formatting, transliteration, string normalization (NFC, NFD, NFKC, NFKD), and the UTF-8 input-to-output pipeline.