i18n-foundations-and-icu

Core internationalization foundations: string externalization strategy, ICU MessageFormat for plurals and gender selects, CLDR plural rules, locale-aware formatting via ECMA-402/Intl APIs, RTL/bidi layout design, Unicode encoding, and text expansion budget planning.

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name	i18n-foundations-and-icu
description	Core internationalization foundations: string externalization strategy, ICU MessageFormat for plurals and gender selects, CLDR plural rules, locale-aware formatting via ECMA-402/Intl APIs, RTL/bidi layout design, Unicode encoding, and text expansion budget planning.

i18n Foundations and ICU MessageFormat

Purpose: give every application the structural foundations it needs to be localized correctly — string externalization, ICU MessageFormat, locale-aware formatting, RTL/bidi, Unicode encoding, and text expansion awareness.

The operating loop

Audit the externalization surface. Grep the codebase for user-facing string literals in render, return, setText, $t, or template expression positions. Every hit is an i18n bug. Scope the extraction: UI strings, error messages, validation messages, notification text, ARIA labels, <title>, and <meta> description — but not log messages, internal IDs, or test fixtures.
Choose a string format and key convention. Select the format for the project type (see the decision tree). Establish a key-naming convention: flat keys (user.profile.title) vs. nested objects ({ user: { profile: { title: "…" } } }). Flat keys are more portable for TMS tools; nested objects map naturally to namespaces. Pick one; document it.
Apply ICU MessageFormat for every message that varies. Plural, gender, and select messages require ICU syntax — not branching in application code.
- Plural: {count, plural, one{# item} other{# items}}. For target locales with more plural categories (Arabic: zero, one, two, few, many, other; Russian: one, few, many, other), provide all required categories. Consult CLDR plural rules at https://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html.
- Gender select: {gender, select, male{He uploaded} female{She uploaded} other{They uploaded}}.
- Ordinal: {position, selectordinal, one{#st} two{#nd} few{#rd} other{#th}}.
- Date/time skeleton: {date, date, ::yMMMd} (CLDR skeleton; not a hard-coded format string).
- Number skeleton: {amount, number, ::currency/USD sign-accounting}.
Replace all new Date().toLocaleString() calls with explicit locale. Pass the user's current locale: new Date().toLocaleString(locale, options). Similarly for Intl.NumberFormat and Intl.RelativeTimeFormat. Never call these without a locale argument.
Audit for RTL/bidi readiness. Check that:
- Layout uses CSS logical properties (margin-inline-start, padding-block-end, inset-inline) rather than physical properties (margin-left, padding-bottom, left).
- The root element sets dir dynamically from the locale: document.documentElement.dir = isRTL ? 'rtl' : 'ltr'.
- Icons and directional images are mirrored for RTL (back arrow, progress indicators).
- Text alignment uses text-align: start not text-align: left.
Audit encoding. Verify UTF-8 end-to-end:
- Source files, build outputs: UTF-8 (no BOM for HTML/CSS/JS).
- Database: utf8mb4 (MySQL) or UTF8 collation (PostgreSQL default). utf8 in MySQL is only 3-byte and cannot store emoji or supplementary characters.
- HTTP headers: Content-Type: text/html; charset=UTF-8.
- HTML <meta charset="UTF-8"> as the first child of <head>.
- Font: verify the target scripts are covered by the font stack.
Plan for 30–50 % text expansion. UI containers should accommodate text up to 150 % of the English source length without truncation or overflow. Use pseudo-localization (see localization-qa-and-pseudo-loc) to verify. Avoid max-width constraints that clip translated text; prefer min-width + overflow: visible or word-break: break-word.

Format and library reference

See the Library/format choice decision tree in ../../knowledge/i18n-l10n-decision-trees.md and the 2026 capability map for current library versions [verify-at-use].

Anti-patterns

String concatenation to build a user-facing message.
if (count === 1) plural handling — correct only for a small set of languages.
toLocaleDateString() / toLocaleString() without a locale argument.
Hard-coded date format strings ("MM/DD/YYYY").
Storing locale-specific formatted values in the database.
Mixing translated text with logic (if (label === "Yes")).

Output

A codebase i18n audit (list of hard-coded strings, missing ICU, bad formatting calls, RTL gaps, encoding issues) + a remediation backlog with effort estimates. Reference ../../templates/string-catalog.md for the catalog scaffold.