Locale-aware interfaces: Intl plural rules, dates and right-to-left layout

Goal

An interface that stays correct when the locale changes: right plural form, right date and number format, right reading direction, no clipped labels; without a separate stylesheet or template per language.

Prerequisites

A BCP 47 locale tag per user or document (lang on html), message strings kept out of code, and the Intl namespace, which MDN lists as providing DateTimeFormat, NumberFormat, PluralRules, RelativeTimeFormat and related constructors.

Steps

1. Numbers and dates: create new Intl.NumberFormat(locale, options) and new Intl.DateTimeFormat(locale, options) once and reuse them; format currencies and units through the options rather than by concatenating symbols. Use Intl.RelativeTimeFormat for "3 days ago". Send ISO 8601 strings over the wire and format only at the edge.
2. Plurals: new Intl.PluralRules(locale).select(n) returns a category (zero, one, two, few, many, other); for English cardinals only one and other occur; some languages use all six. Store one message variant per category and pick by the returned tag; never branch on n === 1. Ordinals (1st, 2nd) use { type: "ordinal" }.
3. Direction: set dir="rtl" on html for right-to-left locales and dir="auto" on inputs that may hold user text of unknown direction. Write layout with logical properties (margin-inline-start, padding-block, inset-inline-end, text-align: start) so margins, floats and alignment flip with the writing direction instead of hard-coding left and right.
4. Icons and controls: mirror direction-bearing icons (back arrows, progress) under [dir="rtl"]; leave universal symbols (play, checkmarks) alone.
5. Space: the W3C text-size article reproduces IBM's expansion table, with short English strings (up to ten characters) expanding by 200–300 % in European languages and long texts by around 130 %. Let buttons, tabs and table headers grow or wrap; avoid fixed widths sized to the English label.
6. Sorting and search: compare with Intl.Collator, not code-point order, and segment text with Intl.Segmenter for character counts.
7. Test with a pseudo-locale (lengthened, accented, mirrored strings) before real translations exist.

Expected result

Switching the locale changes formats, plural forms and layout direction with no code path per language; translators receive complete sentences with named placeholders and one variant per plural category.

Limits and test basis

Behaviour follows the cited MDN and W3C documentation; no measurement is claimed. Time zones and calendars (Hijri, Buddhist) are separate options on DateTimeFormat. Bidirectional text inside a line (a Latin product name in Arabic prose) needs bdi or Unicode isolation, which dir alone does not solve.