Add a new lib-i18n to make message translation easier (#16942)

* Add support to better internationalization with a new lib.

* Add info about `lib-i18n` in the contributing guide.

* Use lib-i18n in M+ as well.

* Change properties files to UTF-8.

lib/i18n/build.gradle.kts

@@ -0,0 +1,21 @@
+plugins {
+    id("com.android.library")
+    kotlin("android")
+}
+
+android {
+    compileSdk = AndroidConfig.compileSdk
+
+    defaultConfig {
+        minSdk = AndroidConfig.minSdk
+        targetSdk = AndroidConfig.targetSdk
+    }
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    compileOnly(libs.kotlin.stdlib)
+}

lib/i18n/src/main/AndroidManifest.xml

@@ -0,0 +1,2 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="eu.kanade.tachiyomi.lib.i18n" />

lib/i18n/src/main/java/eu/kanade/tachiyomi/lib/i18n/Intl.kt

@@ -0,0 +1,83 @@
+package eu.kanade.tachiyomi.lib.i18n
+
+import java.io.InputStreamReader
+import java.text.Collator
+import java.util.Locale
+import java.util.PropertyResourceBundle
+
+/**
+ * A simple wrapper to make internationalization easier to use in sources.
+ *
+ * Message files should be put at the `/res/raw` folder, with the name
+ * `messages_{iso_639_1}.properties`, where `iso_639_1` should be using
+ * snake case and be in lowercase.
+ *
+ * To edit the strings, use the official JetBrain's
+ * [Resource Bundle Editor plugin](https://plugins.jetbrains.com/plugin/17035-resource-bundle-editor).
+ *
+ * Make sure to configure Android Studio to save Properties files as UTF-8 as well.
+ * You can refer to this [documentation](https://www.jetbrains.com/help/idea/properties-files.html#1cbc434e)
+ * on how to do so.
+ */
+class Intl(
+    private val language: String,
+    private val baseLanguage: String,
+    private val availableLanguages: Set<String>,
+    private val classLoader: ClassLoader,
+    private val createMessageFileName: (String) -> String = { createDefaultMessageFileName(it) }
+) {
+
+    val chosenLanguage: String = when (language) {
+        in availableLanguages -> language
+        else -> baseLanguage
+    }
+
+    private val locale: Locale = Locale.forLanguageTag(chosenLanguage)
+
+    val collator: Collator = Collator.getInstance(locale)
+
+    private val baseBundle: PropertyResourceBundle by lazy { createBundle(baseLanguage) }
+
+    private val bundle: PropertyResourceBundle by lazy {
+        if (chosenLanguage == baseLanguage) baseBundle else createBundle(chosenLanguage)
+    }
+
+    /**
+     * Returns the string from the message file. If the [key] is not present
+     * in the current language, the English value will be returned. If the [key]
+     * is also not present in English, the [key] surrounded by brackets will be returned.
+     */
+    operator fun get(key: String): String = when {
+        bundle.containsKey(key) -> bundle.getString(key)
+        baseBundle.containsKey(key) -> baseBundle.getString(key)
+        else -> "[$key]"
+    }
+
+    fun languageDisplayName(localeCode: String): String =
+        Locale.forLanguageTag(localeCode)
+            .getDisplayName(locale)
+            .replaceFirstChar { if (it.isLowerCase()) it.titlecase(locale) else it.toString() }
+
+    /**
+     * Creates a [PropertyResourceBundle] instance from the language specified.
+     * The expected message file will be loaded from the `res/raw`.
+     *
+     * The [PropertyResourceBundle] is used directly instead of [java.util.ResourceBundle]
+     * because the later has issues with UTF-8 files in Java 8, which would need
+     * the message files to be saved in ISO-8859-1, making the file readability bad.
+     */
+    private fun createBundle(lang: String): PropertyResourceBundle {
+        val fileName = createMessageFileName(lang)
+        val fileContent = classLoader.getResourceAsStream(fileName)
+
+        return PropertyResourceBundle(InputStreamReader(fileContent, "UTF-8"))
+    }
+
+    companion object {
+        fun createDefaultMessageFileName(lang: String): String {
+            val langSnakeCase = lang.replace("-", "_").lowercase()
+
+            return "res/raw/messages_$langSnakeCase.properties"
+        }
+    }
+}