Resolved issue where "NewApi" build error occurred in API version 21 …

android/build.gradle

@@ -49,4 +49,6 @@ android {
 
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
+
+    implementation 'org.apache.commons:commons-lang3:3.6'
 }

android/src/main/kotlin/com/miguelruivo/flutter/plugin/countrycodes/country_codes/CountryCodesPlugin.kt

@@ -1,6 +1,7 @@
 package com.miguelruivo.flutter.plugin.countrycodes.country_codes
 
 import android.R
+import android.os.Build
 import java.util.Locale
 import androidx.annotation.NonNull;
 import io.flutter.embedding.engine.plugins.FlutterPlugin
@@ -9,6 +10,7 @@ import io.flutter.plugin.common.MethodChannel
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler
 import io.flutter.plugin.common.MethodChannel.Result
 import io.flutter.plugin.common.PluginRegistry.Registrar
+import org.apache.commons.lang3.LocaleUtils
 
 /** CountryCodesPlugin */
 public class CountryCodesPlugin: FlutterPlugin, MethodCallHandler {
@@ -47,16 +49,75 @@ public class CountryCodesPlugin: FlutterPlugin, MethodCallHandler {
   private fun getLocalizedCountryNames(localeTag: String?) : HashMap<String, String> {
     var localizedCountries: HashMap<String,String> = HashMap()
 
-    val deviceCountry: String = Locale.getDefault().toLanguageTag();
+    val deviceCountry: String = Locale.getDefault().getLanguageTag();
 
     for (countryCode in Locale.getISOCountries()) {
       val locale = Locale(localeTag ?: deviceCountry,countryCode)
-      var countryName: String? = locale.getDisplayCountry(Locale.forLanguageTag(localeTag ?: deviceCountry))
+      var countryName: String? = locale.getDisplayCountry(initLocaleForTag(localeTag ?: deviceCountry))
       localizedCountries[countryCode.toUpperCase()] = countryName ?: "";
     }
     return localizedCountries
   }
 
   override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
   }
+
+  private fun initLocaleForTag(languageTag: String): Locale {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+      return Locale.forLanguageTag(languageTag)
+    }
+
+    return LocaleUtils.toLocale(languageTag)
+  }
+}
+
+private fun Locale.getLanguageTag(): String {
+  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+    return toLanguageTag()
+  }
+
+  // we will use a dash as per BCP 47
+  val SEP = '-'
+  var language: String = language
+  var region: String = country
+  var variant: String = variant
+
+  // special case for Norwegian Nynorsk since "NY" cannot be a variant as per BCP 47
+  // this goes before the string matching since "NY" wont pass the variant checks
+  if (language == "no" && region == "NO" && variant == "NY") {
+    language = "nn"
+    region = "NO"
+    variant = ""
+  }
+
+  if (language.isEmpty() || !language.matches(Regex("\\p{Alpha}{2,8}"))) {
+    language = "und" // Follow the Locale#toLanguageTag() implementation
+    // which says to return "und" for Undetermined
+  } else if (language.equals("iw")) {
+    language = "he" // correct deprecated "Hebrew"
+  } else if (language.equals("in")) {
+    language = "id" // correct deprecated "Indonesian"
+  } else if (language.equals("ji")) {
+    language = "yi" // correct deprecated "Yiddish"
+  }
+
+  // ensure valid country code, if not well formed, it's omitted
+  if (!region.matches(Regex("\\p{Alpha}{2}|\\p{Digit}{3}"))) {
+    region = ""
+  }
+
+  // variant subtags that begin with a letter must be at least 5 characters long
+  if (!variant.matches(Regex("\\p{Alnum}{5,8}|\\p{Digit}\\p{Alnum}{3}"))) {
+    variant = ""
+  }
+
+  val bcp47Tag = StringBuilder(language)
+  if (region.isNotEmpty()) {
+    bcp47Tag.append(SEP).append(region)
+  }
+  if (variant.isNotEmpty()) {
+    bcp47Tag.append(SEP).append(variant)
+  }
+
+  return bcp47Tag.toString()
 }