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Pluralization and ICU Messages

Handling plural rules that differ between languages.

Backend Engineer · Mid-levelFrontend Engineer · Mid-level

Also known as: pluralization, icu messages, plurals i18n

Pluralization selects the right word form by count — and counts work differently per language (English: one/other; French: 0/1 singular; Russian/Arabic: several plural categories). ICU messages encode the rules declaratively: {count, plural, one {# item} other {# items}}, with the runtime picking the locale-correct branch.

ICU: "{n, plural, one {# message} other {# messages}}"
n=1 → "1 message"   n=5 → "5 messages"   (Russian/Arabic pick more branches)

Naive n === 1 ? singular : plural breaks beyond English — and string concatenation breaks translation generally. ICU (via Intl.PluralRules or FormatJS-style libraries) keeps whole sentences translatable with variables, plurals, selects (gender) and number formatting composed.

The classic mistakes:

  • English-only ternaries. count === 1 logic shipped globally mistranslates most languages. ICU plural blocks from the start.
  • Split sentences. "You have" + n + "messages" can’t reorder for languages with different word order. One message, variables inside.
  • Ignoring Intl APIs. Hand-rolled plural maps duplicate Intl.PluralRules badly. Use the platform’s locale data.
  • Zero handling. “0 items” vs “no items” — some locales (and styles) prefer explicit zero forms. Design the zero case deliberately.
  • Gender and selects. Beyond plurals, gendered adjectives and formality need select branches. Collect translator requirements per language, not per English assumption.
  • Untranslated fallback. Missing plural translations shouldn’t crash or blank. Fallback chains to base language, never to nothing.
  • Testing English only. Plural bugs hide until real locales render. Test with plural-rich locales (Russian, Arabic) and pseudolocales in CI.

The rule: whole sentences as ICU messages, plurals/selects/numbers inside, platform locale data underneath. Counts are grammar — conjugate them properly.