How to read from twitch chat for an external app?

Chat and IRC
6

That depends on what programming language you are familiar/comfortable with. My goto suggestion for beginners is PircBotX for Java. After sucessfully setting up this in an enviroment (e.g. Eclipse) you can just replace some of the lines on the page with this:

Configuration configuration = new Configuration.Builder()
    .setName("username") //the twitch username of your bot
    .setServerHostname("irc.twitch.tv")
    .setServerPort(6667)
    .setServerPassword("oauth:xxx") //the ouath token for the same account
    .addAutoJoinChannel("#somechannel") //the channel to connect to
    .addListener(new MyListener())
    .buildConfiguration();

Get the account’s oauth token here.

Edit: forgot for a second you were building an app, so PircBotX (or a library free solution) is what you need for an Android app. I am not familiar with iOS though.