Multilingual voice: building agents that speak to everyone

Only about 20% of the world speaks English, yet most technology is still designed as if everyone does. When voice becomes the interface, that assumption falls apart — because spoken language is messy, nuanced, and deeply tied to culture.

Supporting dozens of languages is easy — doing it well is what matters. At Sierra, our agents have been rigorously evaluated end-to-end across each supported language (34 and counting), testing not just accuracy and latency, but the other signals like rhythm and tone that make speech so powerful.

The complexity of human language

In Thai, tone can change a word’s meaning entirely. What sounds polite in Korean might feel distant to a German speaker. In Portuguese, dialects can differ so much that two native speakers may switch to English to understand each other.

Even fluent multilingual speakers bring accents and patterns that challenge transcription systems built with English assumptions — like fixed word order or phrasing as the only signal of politeness.

Agents built on Sierra address these differences head-on. We’ve learned that the right combination of models — across comprehension, orchestration, reasoning, and generation — varies by locale. Transcription that performs accurately in Japanese might miss nuance in Portuguese, while a synthesis model that sounds natural in Arabic might sound too formal in Hindi.

To capture these nuances, we combine human evaluation with automated benchmarking to measure accuracy, naturalness, and conversational flow. This kind of continuous measurement identifies and deploys the best-performing model combinations for each locale.

Our modular voice architecture keeps the focus where it belongs — on the experience. It handles the complexity of blending and tuning models behind the scenes, allowing each agent to have natural and tuned conversations across the world.

Voices built for the real world

Before any multilingual agent goes live, native speakers test, refine, and vet interactions. Their input ensures agents capture the rhythm and spontaneity of real speech — the kind that synthetic voice data alone can’t reproduce.

That’s why we partner with local language experts in every region where our agents operate. Their feedback shapes how our agents listen, reason, and speak.

Voice Sims build on that foundation, stress-testing patterns at scale — with background noise, interruptions, and complex intents. These large-scale simulations surface edge cases early, ensuring agents are ready for real customers before they ever pick up the phone.

A global wellness brand used this approach to expand its voice agent to over a dozen languages and dialects. Powered by Sierra’s multilingual development framework, the agent handles billing, membership, and support while maintaining the same warmth and tone in every market. Instead of managing separate builds per region, the team deployed one scalable agent that feels local everywhere.

Agents that adapt in real time

Language is dynamic — sometimes even mid-sentence.

A leading delivery platform uses Sierra’s multilingual agent to support its diverse network of partners. The agent can switch languages instantly, helping workers get assistance in whichever language feels most natural — no transfers, no delays.

Because Sierra agents adapt in real time, they don’t just translate; they listen. They detect shifts in tone, sentiment, and language, adjusting phrasing or escalating when needed — just like a great human operator would. That adaptability creates a more inclusive, frictionless experience for everyone involved.

Designing voices that feel local

Our agents don’t simply speak the language; they reflect the tone, rhythm, and personality that make it feel familiar. To capture that nuance, we design and train custom voices that let each brand sound unmistakably itself — in every region it serves.

For a digital bank in Latin America, multilingualism wasn’t about translation — it was about resonance. Working with Sierra, the team developed a specialized voice tuned to regional accents, tone, and personality, ensuring it reflects cultural diversity. The process underscored a key truth: multilingual design is as much about cultural fluency as it is about language itself.

Language as a bridge

Again and again, we’ve seen that multilingual voice agents aren’t just a feature — they’re a bridge to better, more inclusive customer experiences.

For businesses, that bridge opens access to entirely new audiences. For customers, it means being heard — literally — in the language that feels most natural.

And as we all know, to be heard is to be seen.