Language Mixing in Responses

The Problem

AI mixes languages inappropriately—responding in wrong language, switching mid-response, or combining languages confusingly.

Symptoms

• ❌ English query gets Spanish response

• ❌ Response switches languages mid-sentence

• ❌ Technical terms in wrong language

• ❌ Cannot maintain user's language

• ❌ Inconsistent language across conversation

Real-World Example

User query (Spanish): "¿Cómo autenticar la API?"
Knowledge base: Mixed Spanish and English docs

AI response: "Para autenticar, use the API key in el header
Authorization: Bearer {token}. This permite access a los endpoints."

Mixed Spanish/English in single response
Confusing and unprofessional

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Deep Technical Analysis

Language Detection Issues

Query Language Ambiguity:

Short queries hard to detect:
→ "API key" (Universal term)
→ "Configuration" (English? French? Spanish?)

Misdetection:
→ Treats as wrong language
→ Responds inappropriately

Multilingual Content:

Documents contain:
→ English technical terms
→ Local language explanations
→ Code examples (English keywords)

Retrieved context is already mixed:
→ LLM mirrors the mixing
→ Inconsistent output

Code-Switching Behavior

Technical Term Handling:

Many technical terms universal:
→ "API", "OAuth", "JSON", "HTTP"
→ Used across all languages

Question: Should response:
→ Keep English terms? "use the API key"
→ Translate? "use the application programming interface key"
→ Most prefer keeping English technical terms

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How to Solve

Detect query language and set explicit instruction ("Respond in Spanish") + allow English technical terms in non-English responses + use language-tagged chunks with preference boosting + implement language consistency checks post-generation + provide language selector in UI. See Language Handling.