WFGY/ProblemMap/GlobalFixMap/Language/locale_drift.md

Locale Drift — Guardrails and Fix Patterns

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Stabilize retrieval when locale settings silently change token rules, analyzers, and normalization between ingest, index, and query. Typical failures include en_US vs en_GB spelling, tr_TR case-folding (“i/İ”), decimal and thousands separators, date formats, Simplified/Traditional Chinese, and accent stripping differences.

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Open these first

• Visual map and recovery: rag-architecture-and-recovery.md
• End to end retrieval knobs: retrieval-playbook.md
• Why this snippet and how to cite: retrieval-traceability.md
• Snippet schema fence: data-contracts.md
• Embedding vs meaning: embedding-vs-semantic.md
• Chunk boundary sanity: chunking-checklist.md

Related in this folder:

• Tokenizer drift: tokenizer_mismatch.md
• Mixed scripts in one query: script_mixing.md

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Core acceptance targets

• ΔS(question, retrieved) ≤ 0.45 across locale variants of the same query
• Coverage of the target section ≥ 0.70 after repair
• λ remains convergent across three paraphrases and two seeds
• E_resonance flat on long windows that include locale-sensitive tokens (dates, numbers, currencies)

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What this failure looks like

Symptom	Likely cause	Where to fix
High similarity yet wrong section when query switches , and . in numbers (e.g., 1.234,56)	Different locale decimal/thousand separators between ingest and query	Normalize numerics before index and query; align analyzers
Dates “03/07/2024” retrieved as July instead of March	Ambiguous locale date parsing	Canonicalize to ISO YYYY-MM-DD at ingest and query
“istanbul” mismatches titles with “İstanbul”	Turkish case-folding rules differ across stages	Use locale-aware fold or ASCII base form consistently
“straße/strasse” flip in German content	ß vs ss normalization mismatch	Decide policy (preserve ß or fold to ss) and apply everywhere
“café” differs from “cafe” across stores	Accent stripping only on one side	Apply accent policy uniformly; prefer keeping both forms via subfield
English vs Chinese punctuation causes token joins/drops	Locale-specific punctuation width and spacing	Normalize width, unify punctuation rules; ensure same analyzer
zh-Hans vs zh-Hant documents never co-retrieve	Variant mapping missing	Map variants at ingest or add alias field; verify embeddings share policy

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Fix in 60 seconds

1. Measure ΔS Compute ΔS(question, retrieved) with current locale. Re-run with a canonicalized query: ISO dates, normalized numbers, consistent case-folding. If ΔS drops by ≥ 0.10, locale drift is your root cause.

2. Probe λ_observe Flip only the locale-sensitive tokens (date, number, currency symbol, diacritics). If λ flips or citations jump, lock schema and fix normalization before touching rerankers.

3. Apply the smallest structural change

• Canonical numerics: convert decimals to . and thousands to thin-space or remove thousands.
• Canonical dates: rewrite to YYYY-MM-DD and store a parsed date field for filters.
• Case-folding: choose locale-aware rules where needed (tr_TR i/İ), else use simple lower with exceptions list.
• Diacritics: either preserve and add an accent-folded subfield, or fold everywhere.
• CJK: unify Simplified/Traditional mapping per field and keep a raw subfield.

4. Verify Coverage ≥ 0.70 and ΔS ≤ 0.45 on three paraphrases and two seeds using both locale renderings.

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Minimal repair recipes by stack

Elasticsearch / OpenSearch

• Define a canonical analyzer chain shared by index and search_analyzer. Suggested: ICU normalizer (NFC) → width fold → optional accent fold (or keep + keyword subfield) → locale-aware lowercase.
• Add numeric and date normalizers in an ingest pipeline. Persist ISO strings, plus typed fields for range queries.
• For German and Turkish, use dedicated token filters (german_normalization, custom fold for Turkish i/İ).
• For Chinese, Japanese: keep a keyword subfield for exact product names and a bigram analyzer for recall. Open: retrieval-playbook.md

BM25 in code or light stores

• Pre-normalize text and queries with a single code path: ISO dates, canonical numerics, consistent punctuation width, optional accent fold, locale-aware lower.
• Log the effective tokens to verify identical behavior across runs. Open: retrieval-traceability.md

Vector stores (FAISS, Milvus, Qdrant, Weaviate, pgvector)

• Apply the same locale normalization before embedding for both corpus and queries.
• For numerics and dates, consider lexical sidecar (BM25) to capture exact forms, then deterministic rerank.
• Re-embed a gold slice to validate ΔS before full rebuild. Open: pattern_vectorstore_fragmentation.md

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Locale normalization policy — quick checklist

• Dates stored and queried as ISO YYYY-MM-DD; display can be localized later.
• Numerics use . as decimal; thousands removed or unified; currency symbol separated from amount.
• Case-folding policy documented; Turkish special-case applied where needed.
• Accent policy consistent: preserve + accent-folded subfield, or fold globally.
• CJK variant policy decided (Hans/Hant) and applied at both ingest and query.
• Punctuation width unified; zero-width and bidi controls stripped where not meaningful.
• Analyzer identity enforced across index and search paths.

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Diagnostic checklist

• Same normalization code runs in ingest and in query clients.
• Same analyzer configuration used for the field in both index_analyzer and search_analyzer.
• Logging proves that effective tokens match across locales for the same meaning.
• Citations remain in the same section after locale canonicalization.
• Rerank stage reads normalized text, not raw payloads.

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Copy-paste tests

Locale flip probe

Q0: original user query as typed (locale A)
Q1: ISO date, canonical number, same words (locale neutral)
Q2: render in locale B (date/number style only)

Return ΔS for Q0,Q1,Q2, λ_state per run, and a note if citations left the target section.

Turkish i/İ sanity

Build two forms: 'istanbul', 'İstanbul'.
Verify tokens and matches are identical against titles and anchor fields.
If not, log analyzer outputs and apply a Turkish-aware fold.

Accent policy audit

Index doc: 'café', 'résumé'.
Queries: 'cafe', 'resume', original diacritics.
Expect both forms to match the same snippet and citations to remain stable.

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When to escalate

• ΔS stays ≥ 0.60 after locale normalization and analyzer alignment. Re-chunk with stable boundaries and re-embed a gold slice. Open: chunking-checklist.md

• Answers alternate between locales while citations drift. Enforce snippet schema and forbid cross-section reuse. Open: data-contracts.md, retrieval-traceability.md

• Hybrid retrieval still underperforms a single retriever after fixes. Align locale rules before rerank and make rerank deterministic. Open: rerankers.md

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