A Babel fish you can actually trust, with caveats

What it does RTranslator is an Android app that turns your phone into a real-time voice translator. It listens, transcribes with OpenAI’s Whisper, translates with Meta’s NLLB, and speaks the result—all on-device. Two modes matter: Conversation links two phones over Bluetooth LE so each person hears their own language through headphones, while WalkieTalkie handles quick exchanges on a single phone with turn-taking.

The interesting bit The author didn’t just slap models into an app. He converted NLLB and Whisper to ONNX, quantized to int8, split model weights to deduplicate RAM at runtime, and added KV caching. The result squeezes a 600M-parameter translator and 244M-parameter recognizer into roughly 6GB of phone RAM. It’s the kind of optimization work that sounds tedious until you realize it’s what makes offline translation not crash your phone.

Key highlights

• Runs fully offline after initial 1.2GB model download; no server, no data collection
• Conversation mode supports multi-party mesh via Bluetooth LE
• Background operation with screen off (though some OEMs throttle this)
• 31 languages at full quality, plus dozens more at reduced quality via settings toggle
• Version 3.0 funded by NLnet’s Mobifree program promises fully open-source stack and better models by 2026

Caveats

• Not fully open source yet: Google’s ML Kit handles language detection in WalkieTalkie mode, and NLLB carries a non-commercial license
• Requires at least 6GB RAM and a fast CPU; the author bluntly suggests using v1.0 “if you have a pretty crappy phone”
• System TTS quality varies wildly by device; Google TTS is recommended but not bundled

Verdict Grab this if you need private, offline translation and carry a reasonably modern Android phone. Skip it if you’re on budget hardware, need commercial use rights today, or demand iOS support.