<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="app_name">Nome do App</string>
    <string name="action_settings">Settings</string>
    // Demais strings
</resources>

String appName = context.getString(R.string.app_name);

<TextView
    android:id="@+id/text_view"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/app_name" /> <!-- Aqui você referencia a String da chave app_name.

- /res/
    - /values/
        - strings.xml
        - outros...
    - /values-pt/
        - strings.xml
        - outros...
    - /values-fr/
        - strings.xml
        - outros...
    - Demais resources