LangX | Practice, Learn, Succeed!

LangX connects you with native speakers so you can practise the language you are learning by teaching the one you already know. Speak, chat, correct each other's sentences — and keep a streak while doing it.

iOS, Android and the web all come out of one Expo codebase, with a single Fastify API behind them. BSD-3, open source, same as v1.

Discover — people learning your language, nearby and active Chat — say it wrong, get it fixed Tokens — earned by teaching

_{Screens are rendered from the app's own layouts with demonstration data; the faces are AI-generated portraits. The full press kit is in langx/branding.}

Official Links

:computer: Website: https://langx.io/

:calling: Apps: https://get.langx.io

:coin: Token Website: https://token.langx.io/

:scroll: Litepaper: https://docs.langx.io

:bar_chart: Insights: https://insight.langx.io

:green_circle: Status : https://status.langx.io/

:wrench: Backlog: https://backlog.langx.io/

:envelope: Mail: [email protected]

Tech Stack

| Layer | Choice | | ---------- | ------------------------------------------------------------------- | | Mobile/web | Expo SDK 57 · expo-router · TanStack Query + Zustand · NativeWind | | API | Node 24 · Fastify 5 · Better Auth · Socket.io — a single container | | Data | MongoDB Atlas, official driver (no Mongoose) | | Storage | S3-compatible (Backblaze B2 in production; Cloudflare R2 works too) | | Billing | RevenueCat — StoreKit / Play Billing / RevenueCat Web + Stripe |

Other Repos

What each one is, where it is hosted and which values are copied between them by hand: docs/repo-map.md.

🟢 website _SvelteKit — langx.io_
🟢 token-website _HTML/CSS — token.langx.io_
🟢 docs _GitBook source — docs.langx.io_
🟢 branding _logos, icons, store artwork,

press kit_

🟡 copilot _Discord assistant; live, not

developed_

Previous versions

🌱 v1 branch — this repository before

v2; the Expo rewrite that was never released

📦️ api (Archived) _v1's Express API; served

api.langx.io until v2 took the hostname_

📦️ langx-angular (Archived) _Originally

developed; v0.15, the last v1 on the stores, was built from it_

📦️ langx-flutter (Archived) _(Not

completed)_

Get started

MongoDB has to run as a replica set — Better Auth wraps its writes in transactions, which standalone mongod does not support. Full setup, including the one-line Docker version, is in CONTRIBUTING.md.

pnpm install
cp .env.example .env      # set MONGODB_URI and BETTER_AUTH_SECRET
pnpm dev                  # API on :4000, Expo on :8081

pnpm dev:api — API only
pnpm dev:web — Expo web only
pnpm typecheck · pnpm lint · pnpm test

Press i or a in the Expo output for a simulator, or open http://localhost:8081 for the web build. Running on a physical phone needs a development build rather than Expo Go — see CONTRIBUTING.md.

Layout

apps/api          Fastify API — auth, REST, realtime, scheduled jobs
apps/mobile       Expo app (iOS + Android + web)
packages/shared   zod schemas, language + level tables, PLAN_LIMITS, TOKEN_RULES

docs/architecture.md describes how they fit together and docs/decisions.md says why — several choices look arbitrary until you know what went wrong the first time.

Self-hosting

Your own instance needs MongoDB, an S3-compatible bucket and an email key. Billing, translation and analytics are optional and degrade cleanly when they are not configured. See docs/self-host.md.