This is a list of all the respositories listed under the "acknowledgements" link at the bottom of the splatoon pages in the the app, and settings menu from the main page of the app. Since those pages only list the licenses, I have compiled a list with the github/npm links.
Based on the open source dependencies, I think we can safely assume that the Nintendo is using Node for the back-end API, and React for the front-end web view in the Splatoon section of the app. The main app itself is built using Xamarin (C#). Opus is used as the codec for the voice chat features.
- Echo Cancellation Program
- libSRTP (Note: Using version commited before Mar 26, 2017)
- Opus
- WebRTC AudioProcessing
- Breakpad
- Curl
- Libdisasm
- gflags
- Linux SysCall Support
- musl
- CarouselView.FormsPlugin
- Lottie
- Plugins for Xamarin
- Hockey SDK
- Math.NET Numerics
- Json.NET
- PCLCrypto
- P/Invoke
- Share Plugin
- Validation
- Xamarin Forms Lab
- Xamarin SDK
- FFImageLoading
- SkiaSharp
- WebP.Touch
- ModernHttpClient
- ASAP
- babel-polyfill
- babel-runtime
- chain-function
- consolidated-events
- core-js (Note: Using version commited before Feb 24, 2016)
- create-react-class
- deep-equal
- dom-helpers
- encoding
- exenv
- fbjs
- history
- hoist-non-react-statics
- iconv-lite
- immutability-helper
- immutable
- invariant
- is-stream
- isarray
- isomorphic-fetch
- js-tokens
- lodash
- lodash-es
- loose-envify
- moment
- mustache
- node-fetch
- object-assign
- path-to-regexp
- promise
- prop-types
- react
- react-dom
- react-helmet
- react-redux
- react-router
- react-router-dom
- react-router-redux
- react-side-effect
- react-transition-group
- react-waypoint
- reduce-reducers
- redux
- redux-actions
- redux-saga
- regenerator-runtime
- resolve-pathname
- setimmediate
- shallowequal
- symbol-observable
- ua-parser-js
- url-search-params
- uuid
- value-equal
- warning
- whatwg-fetch