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Documentation Index

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Nixflex supports voice agents in 15+ languages. The engine uses a hybrid STT routing strategy and matches the caller’s language automatically.

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How language is decided

The agent’s language field is the default — what the agent speaks in if the caller’s language can’t be inferred. In practice, language is selected as follows:
1

Caller wins

If the caller speaks a language clearly, the agent matches them — even if language is set to something else.
2

Country prefix hint

For inbound calls from GCC region numbers (+971, +966, +974, +965, +973, +968), the engine routes to Deepgram for Arabic-first transcription.
3

Multilingual STT

For all other regions, AssemblyAI’s multilingual model handles 6 major languages (EN, ES, FR, DE, IT, PT) with auto-detection.
4

Agent default

If detection is ambiguous, the agent’s configured language is used.

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Supported languages

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Voice (TTS)

15 languages via Inworld TTS Max:
LanguageCodeNotes
English (US/UK/AU)enMultiple voice options
ArabicarModern Standard Arabic
Spanishes
Frenchfr
Germande
Italianit
Portuguesept
Dutchnl
Polishpl
Russianru
Hindihi
Koreanko
Japaneseja
ChinesezhMandarin
Turkishtr

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Speech recognition (STT)

  • AssemblyAI multilingual — 6 languages with real-time auto-detection (EN, ES, FR, DE, IT, PT)
  • Deepgram Nova 3 — used for GCC region numbers; full Arabic support, 36+ other languages
The engine picks the right STT provider at call start based on the destination number.

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Locking the language

By default the agent matches the caller. To force a specific language regardless of what the caller speaks, add this line to your system prompt: 
Always respond in English, even if the caller speaks another language.
Claude will honour the explicit instruction.
Locking is useful when your business operates in one language only and you don’t want the agent to attempt translations it might get wrong.

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Backchannels and fillers

The engine speaks small filler words while listening — “mm-hmm”, “I see”, “right” — to make the call feel natural. These match the caller’s language automatically across all 15 supported languages.

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Numbers and pronunciation

Prices, phone numbers, dates, and addresses are normalised before being sent to the TTS. The agent says “twenty-five pounds” rather than spelling “two-five-pound-sign”. This works across languages without configuration. For brand or technical pronunciations the agent gets wrong, you can override them in the system prompt: 
Pronunciation: pronounce "Nixflex" as "NIKS-flex". Say "Anthropic" as "an-THROP-ik".

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What’s not supported yet

  • Voice cloning per language (planned)
  • Per-call language switching mid-conversation (the call locks to one language at start)
  • Languages outside the 15 TTS list above