Internationalization file checker

About this tool

Compare a reference locale with a target locale before releasing translated messages. The checker retains nested object paths, empty containers and JSON types, then reports missing keys, extra keys and mismatches at JSON Pointer locations. Choose plain text, named braces or the explicitly limited ICU-flat grammar; no template is rendered or executed. Translation wording and primitive values are deliberately not compared. The result is a structural review aid, not a translation-quality score or proof that your localization framework will accept every message.

Reproducible localization checks

reference:
{"greeting":"Hello {name}","count":1,"enabled":true,"empty":{}}

target:
{"greeting":"こんにちは {broken","count":99,"enabled":false,"empty":{}}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[]}

reference:
{"menu":{"home":"Home","gone":"Old"},"count":1,"empty":{}}

target:
{"menu":{"home":"首页","extra":"New"},"count":"1","empty":[]}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"indices"}

{"valid":true,"issues":[{"code":"missing-key","path":"/menu/gone","side":"before","severity":"error"},{"code":"type-mismatch","path":"/count","side":"after","severity":"error"},{"code":"type-mismatch","path":"/empty","side":"after","severity":"error"},{"code":"extra-key","path":"/menu/extra","side":"after","severity":"error"}]}

reference:
{"group":{"title":"Title"},"empty":{}}

target:
{}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"missing-key","path":"/group","side":"before","severity":"error"},{"code":"missing-key","path":"/group/title","side":"before","severity":"error"},{"code":"missing-key","path":"/empty","side":"before","severity":"error"}]}

reference:
{"a/b":{"~name":"x"},"a.b":"x","__proto__":"text","constructor":{}}

target:
{"a/b":{},"a.b":"y","__proto__":"译文","constructor":{}}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"missing-key","path":"/a~1b/~0name","side":"before","severity":"error"}]}

reference:
{"title":"One","ti\u0074le":"Two"}

target:
{}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":false,"issues":[{"code":"duplicate-key","path":"/title","side":"before","severity":"error"}]}

reference:
{"steps":["A","B"]}

target:
{"steps":["一"]}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":false,"issues":[{"code":"arrays-rejected","path":"/steps","side":"before","severity":"error"},{"code":"arrays-rejected","path":"/steps","side":"after","severity":"error"}]}

reference:
{"steps":["A","B"]}

target:
{"steps":["一"]}

options:
{"mode":"plain","beforeLocale":"en","afterLocale":"ja","arrays":"indices"}

{"valid":true,"issues":[{"code":"missing-key","path":"/steps/1","side":"before","severity":"error"}]}

reference:
{"label":"{{literal}} {name} {0} {name}"}

target:
{"label":"{0} {name} {{文字}}"}

options:
{"mode":"braces","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[]}

reference:
{"label":"Hello {name}, item {0}"}

target:
{"label":"{user} さん、項目 {0}"}

options:
{"mode":"braces","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"placeholder-mismatch","path":"/label","side":"after","severity":"error"}]}

reference:
{"label":"Hello {name"}

target:
{"label":"こんにちは {name}"}

options:
{"mode":"braces","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"message-syntax","path":"/label","side":"before","severity":"error"}]}

reference:
{"cart":"{count, plural, one {# item} other {# items}}"}

target:
{"cart":"{count, plural, other {# 個}}"}

options:
{"mode":"icu-flat","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[]}

reference:
{"cart":"{count, plural, other {# items}}"}

target:
{"cart":"{count, plural, one {# 個} other {# 個}}"}

options:
{"mode":"icu-flat","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"category-coverage","path":"/cart","side":"before","severity":"warning"},{"code":"category-unreachable","path":"/cart","side":"after","severity":"warning"}]}

reference:
{"cart":"{count, plural, =0 {None} one {# {unit}} other {# {unit}}}"}

target:
{"cart":"{count, plural, other {# 個}}"}

options:
{"mode":"icu-flat","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"plural-placeholder-mismatch","path":"/cart","side":"after","severity":"error"},{"code":"branch-placeholder-mismatch","path":"/cart","side":"after","severity":"error"},{"code":"exact-selector-missing","path":"/cart","side":"after","severity":"warning"}]}

reference:
{"nested":"{n, plural, one {{m, plural, other {x}}} other {x}}","format":"{price, number}","required":"{n, plural, one {x}}"}

target:
{"nested":"{n, plural, other {#}}","format":"{price}","required":"{n, plural, other {#}}"}

options:
{"mode":"icu-flat","beforeLocale":"en","afterLocale":"ja","arrays":"reject"}

{"valid":true,"issues":[{"code":"unsupported-nested","path":"/nested","side":"before","severity":"error"},{"code":"unsupported-formatter","path":"/format","side":"before","severity":"error"},{"code":"missing-other","path":"/required","side":"before","severity":"error"}]}

Common localization review mistakes

• Choosing braces for an i18next-style {{name}} message: in this tool’s braces mode that spelling is literal text, not an argument.
• Comparing language-specific plural category names as if every language needed the English set.
• Using dotted paths for keys that actually contain dots, slashes or tildes instead of reading the escaped JSON Pointer.
• Enabling index comparison for arrays whose elements have been reordered; the checker does not match elements by identity.
• Treating a report with valid: true as error-free, or a filtered screen as the entire exported report.
• Sharing an issue report without reviewing private key paths and argument names; translation values are omitted, but names can still identify a project.

Limits and notes

• —Only strict JSON with object roots is accepted: no JSONC comments, trailing commas, YAML, JavaScript modules or automatic repair. Duplicate decoded keys and unpaired Unicode surrogates are rejected, including at nested paths. Object-key order is ignored; names are case-sensitive and are not Unicode-normalized. JSON Pointer escapes ~ as ~0 and / as ~1, so dotted keys are not confused with nesting; __proto__ and constructor remain ordinary data keys. Arrays are rejected by default or explicitly compared by zero-based index. Empty objects, empty arrays and null retain distinct types.
• —Plain mode ignores all message grammar. Braces mode recognizes {name} or positional {0}, with ASCII identifiers [A-Za-z_][A-Za-z0-9_]* or digits only; {{ and }} are literal brace escapes. Malformed single braces are errors. Argument sets are compared, so reordering or repeating the same argument is allowed. Values, translation wording and repeated-argument counts are not diffed.
• —ICU-flat supports simple arguments, one-level plural and selectordinal with named categories or nonnegative integer selectors such as =0. Branches may contain simple arguments and # text. Nested selectors, select, formatters such as number/date, offset, repeated plural definitions for one argument, and ICU apostrophe escaping are unsupported and diagnosed. Each plural requires other. Supported syntax is inspected without formatting or evaluating any message; this is not full ICU MessageFormat compatibility. Inside a plural branch, # counts as a use of its controlling argument, equivalent to {count} for a plural controlled by count; outside a plural it is literal text. In ICU mode, numeric argument identifiers are canonical integers 0–32767, with no leading zeros; this includes plural-controller names.
• —Plural categories come from the current runtime’s Intl.PluralRules for each supplied locale and cardinal/ordinal type. Missing optional categories and categories unreachable for that locale are warnings, not a demand for identical categories across languages. Exact-selector differences are also warnings. A target branch compares arguments with the same reference selector, or with reference other when no selector matches. Fallback and coverage checks are static approximations; they do not establish linguistic correctness or runtime rendering behavior. The union of arguments across all plural branches is also compared, so one lost argument can produce both plural-wide and branch-level errors. Exact =N branches may spell their fixed number, so only their controlling argument is excluded from branch and union placeholder comparisons; other names remain checked.
• —Each input is limited to 262,144 UTF-8 bytes, depth 32 and 10,000 JSON nodes; each message has a 1,000-token argument/branch budget. At most 1,000 issues and a 1 MiB (1,048,576-byte) JSON report are retained. A truncated or oversized report cannot be exported as complete. Invalid JSON prevents comparison/export; supported JSON with message errors can still export diagnostics. Processing is local, with no uploads, network lookup, persistence or expression execution. Reports omit translation values but retain key paths and argument names, which may themselves be private. Each JSON Pointer is capped at 4,096 UTF-8 bytes and all paths together at 1 MiB; a diagnostic detail over 512 UTF-16 code units marks the report truncated and blocks export.

Frequently asked questions

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