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Ecma-402 has a specification for Lookup Matching, described by LookupMatcher (9.2.3) and BestAvailableLocale (9.2.2). It's based on the Lookup algorithm described in RFC 4647 section 3.4. But, both Ecma-402 specification and RFC 4647 algorithm fail to perform such task in some cases. For example:

  • az-IR maps to az (default for az-Latn-AZ), not to the correct az-Arab-IR.
  • en-Latn-GB and en-Latn-IN map to en (default for en-Latn-US), not to the correct en-GB and en-IN respectively.
  • ha-CM and ha-SD map to ha (default for ha-Latn-NG), not the correct ha-Arab-*.
  • kk-CN, kk-IR, and kk-MN map to kk (default for kk-Cyrl-KZ), not the correct kk-Arab-*.
  • sr-ME, sr-RO, sr-RU, sr-TR map to sr (default for sr-Cyrl-RS), not to the correct sr-Latn-*.
  • uz-AF maps to uz (default for uz-Latn-UZ), not to the correct uz-Arab-AF.
  • zh-AU, zh-BN, zh-GB, zh-GF, zh-ID, zh-MO, zh-MY, zh-PA, zh-PF, zh-PH, zh-SR, zh-TH, zh-US, and zh-VN map to zh (default for zh-Hans-CN), not to the correct zh-Hant-*.


The wrong specification can be observed in today's implementations.

// Both bugs bellow happen on latest Firefox and Chrome.

new Intl.NumberFormat("az").format(NaN); // "NaN" (as expected)
new Intl.NumberFormat("az-IR").format(NaN); // "NaN" (wrong, Arab expected)

new Intl.NumberFormat("en-IN").format(123456789); // "12,34,56,789" (as expected)
new Intl.NumberFormat("en-Latn-IN").format(123456789); // "123,456,789" (bypasses en-IN)

new Intl.DateTimeFormat("en-GB").format(new Date()); // "13/01/2015" (as expected)
new Intl.DateTimeFormat("en-Latn-GB").format(new Date()); // "1/13/2015" (bypasses en-GB)

new Intl.NumberFormat("ha-CM").format(NaN); // "NaN" (wrong, Arab expected)
new Intl.NumberFormat("ha-SD").format(NaN); // "NaN" (wrong, Arab expected)

new Intl.NumberFormat("kk-AF").format(NaN); // "NaN" (wrong, Arab expected)
new Intl.NumberFormat("kk-CN").format(NaN); // "NaN" (wrong, Arab expected)
new Intl.NumberFormat("kk-IR").format(NaN); // "NaN" (wrong, Arab expected)

new Intl.DateTimeFormat("sr", {month: "long"}).format(new Date()); // "фебруар" (as expected)
new Intl.DateTimeFormat("sr-Latn", {month: "long"}).format(new Date()); // "februar" (as expected)
new Intl.DateTimeFormat("sr-ME", {month: "long"}).format(new Date()); // "februar" (as expected)
new Intl.DateTimeFormat("sr-ME", {month: "long"}).format(new Date()); // "фебруар" (wrong, Latn expected)
new Intl.DateTimeFormat("sr-RO", {month: "long"}).format(new Date()); // "фебруар" (wrong, Latn expected)
new Intl.DateTimeFormat("sr-RU", {month: "long"}).format(new Date()); // "фебруар" (wrong, Latn expected)
new Intl.DateTimeFormat("sr-TR", {month: "long"}).format(new Date()); // "фебруар" (wrong, Latn expected)

new Intl.NumberFormat("uz").format(NaN); // "NaN" (as expected)
new Intl.NumberFormat("uz-AF").format(NaN); // "NaN" (wrong, Arab expected)

new Intl.NumberFormat("zh").format(NaN); // "NaN" (as expected)
new Intl.NumberFormat("zh-AU").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-BN").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-CN").format(NaN); // "NaN" (as expected)
new Intl.NumberFormat("zh-GB").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-GF").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-HK").format(NaN); // "非數值" (as expected)
new Intl.NumberFormat("zh-ID").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-MO").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-MO").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-MY").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-PA").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-PF").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-PH").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-SR").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-TH").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-TW").format(NaN); // "非數值" (as expected)
new Intl.NumberFormat("zh-US").format(NaN); // "NaN" (wrong, Hant expected)
new Intl.NumberFormat("zh-VN").format(NaN); // "NaN" (wrong, Hant expected)


The algorithm specified is too simplistic. It basically instruct implementations to perform subtags truncation until it finds a locale, which is wrong. Related issues and more details can be found on:


According to Mark Davis, "the recommended methodology for Bundle lookup is to use Language Matching". Therefore, there are two options:

  1. Follow the Language Matching algorithm specified by the Unicode Technical Standard #35.
  2. Follow the quicker algorithm from cldrjs, which produces the same result as the Language Matching considering a score threshold of 100%.