Android application development, switch display language within the application

Category：Android

Pub.date：2021-03-16

How to switch the display language within the app in Android app development. 
For example, if you want to display English on the device even in a Japanese environment.

Android Studio Version 4.1.2 
Kotlin 1.4.31

## Part of the code

```
class MainActivity : AppCompatActivity() {
    private var localeLanguage: String = ""		// "" or "en" or "jp" or etc.

companion object {
        internal const val SETTINGS: String = "settings"
        internal const val LOCALE_LANGUAGE: String = "localeLanguage"
    }

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }

//Save localeLanguage
	//Save a string such as "en" or "ja" with some action
	//You may restart Activity by executing recreate() at the timing of saving.
    private fun saveLocaleLanguage() {
        getSharedPreferences(SETTINGS, Context.MODE_PRIVATE).edit().apply {
            putString(LOCALE_LANGUAGE, this@MainActivity.localeLanguage)
            apply()
        }
    }

//Language setting
	//Switch the language when the Activity starts.
	//Prepare as many strings.xml as you need.
    override fun attachBaseContext(base: Context) {
        val pref = base.getSharedPreferences(SETTINGS, Context.MODE_PRIVATE)
        this.localeLanguage = pref.getString(LOCALE_LANGUAGE, "") ?: ""
        val loc: Locale? = if (this.localeLanguage != "") Locale(this.localeLanguage) else null
        if (loc != null) {
            val res = base.resources
            val config = Configuration(res.configuration)
            val localeList = LocaleList(loc)
            LocaleList.setDefault(localeList)
            config.setLocales(localeList)
            super.attachBaseContext(base.createConfigurationContext(config))
        } else {
            super.attachBaseContext(base)
        }
    }

}
```

## Prepare as many strings.xml as you need

An example of preparing all 11 languages with English as the default

main/res/values/strings.xml 
main/res/values-de/strings.xml 
main/res/values-es/strings.xml 
main/res/values-fr/strings.xml 
main/res/values-it/strings.xml 
main/res/values-ja/strings.xml 
main/res/values-nl/strings.xml 
main/res/values-pl/strings.xml 
main/res/values-pt/strings.xml 
main/res/values-ru/strings.xml 
main/res/values-zh/strings.xml

The arguments given to Locale() are as follows. 
"en" 
"de" 
"es" 
"fr" 
"it" 
"ja" 
"nl" 
"pl" 
"pt" 
"ru" 
"zh"

## Support for SDK Version 23 and below

```
    //Language setting
    override fun attachBaseContext(base: Context) {
        val pref = base.getSharedPreferences(SETTINGS, Context.MODE_PRIVATE)
        this.localeLanguage = pref.getString(LOCALE_LANGUAGE, "") ?: ""
        val loc: Locale? = if (this.localeLanguage != "") Locale(this.localeLanguage) else null
        if (loc != null) {
            val res = base.resources
            val config = Configuration(res.configuration)
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {    //minSdkVersion 24
                val localeList = LocaleList(loc)
                LocaleList.setDefault(localeList)
                config.setLocales(localeList)
            } else {    //minSdkVersion 17
                config.setLocale(loc)
            }
            super.attachBaseContext(base.createConfigurationContext(config))
        } else {
            super.attachBaseContext(base)
        }
    }
```