package com.learnium.RNDeviceInfo;

import javax.annotation.Nullable;

import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.bridge.ReactContextBaseJavaModule;
import com.facebook.react.bridge.ReactMethod;
import com.facebook.react.bridge.ReadableMap;
import com.facebook.react.bridge.ReadableArray;
import com.facebook.react.bridge.Callback;

import android.os.Build;
import android.provider.Settings.Secure;
import android.content.pm.PackageManager;
import android.content.pm.PackageInfo;
import android.bluetooth.BluetoothAdapter;

import java.util.HashMap;
import java.util.Map;

public class RNDeviceModule extends ReactContextBaseJavaModule {

  ReactApplicationContext reactContext;

  public RNDeviceModule(ReactApplicationContext reactContext) {
    super(reactContext);
    this.reactContext = reactContext;
  }

  @Override
  public String getName() {
    return "RNDeviceInfo";
  }

  @Override
  public @Nullable Map<String, Object> getConstants() {
    HashMap<String, Object> constants = new HashMap<String, Object>();

    PackageManager packageManager = this.reactContext.getPackageManager();
    String packageName = this.reactContext.getPackageName();

    constants.put("appVersion", "not available");
    constants.put("buildNumber", 0);

    try {
      PackageInfo info = packageManager.getPackageInfo(packageName, 0);
      constants.put("appVersion", info.versionName);
      constants.put("buildNumber", info.versionCode);
    } catch (PackageManager.NameNotFoundException e) {
      e.printStackTrace();
    }

    String deviceName = "Unknown";

    try {
      BluetoothAdapter myDevice = BluetoothAdapter.getDefaultAdapter();
      deviceName = myDevice.getName();
    } catch(Exception e) {
      e.printStackTrace();
    }

    constants.put("deviceName", deviceName);
    constants.put("systemName", "Android");
    constants.put("systemVersion", Build.VERSION.RELEASE);
    constants.put("model", Build.MODEL);
    constants.put("deviceId", Build.BOARD);
    constants.put("deviceLocale", this.reactContext.getResources().getConfiguration().locale.toString());
    constants.put("uniqueId", Secure.getString(this.reactContext.getContentResolver(), Secure.ANDROID_ID));
    constants.put("systemManufacturer", Build.MANUFACTURER);
    constants.put("bundleId", packageName);
    constants.put("userAgent", System.getProperty("http.agent"));
    return constants;
  }
}

package com.learnium.RNDeviceInfo;

import android.bluetooth.BluetoothAdapter;
import android.content.pm.PackageInfo;
import android.content.pm.PackageManager;
import android.os.Build;
import android.provider.Settings.Secure;

import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.bridge.ReactContextBaseJavaModule;

import java.util.HashMap;
import java.util.Locale;
import java.util.Map;

import javax.annotation.Nullable;

public class RNDeviceModule extends ReactContextBaseJavaModule {

  ReactApplicationContext reactContext;

  public RNDeviceModule(ReactApplicationContext reactContext) {
    super(reactContext);
    this.reactContext = reactContext;
  }

  @Override
  public String getName() {
    return "RNDeviceInfo";
  }

  private String getCurrentLanguage() {
      Locale current = getReactApplicationContext().getResources().getConfiguration().locale;
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
          return current.toLanguageTag();
      } else {
          StringBuilder builder = new StringBuilder();
          builder.append(current.getLanguage());
          if (current.getCountry() != null) {
              builder.append("-");
              builder.append(current.getCountry());
          }
          return builder.toString();
      }
  }

  @Override
  public @Nullable Map<String, Object> getConstants() {
    HashMap<String, Object> constants = new HashMap<String, Object>();

    PackageManager packageManager = this.reactContext.getPackageManager();
    String packageName = this.reactContext.getPackageName();

    constants.put("appVersion", "not available");
    constants.put("buildNumber", 0);

    try {
      PackageInfo info = packageManager.getPackageInfo(packageName, 0);
      constants.put("appVersion", info.versionName);
      constants.put("buildNumber", info.versionCode);
    } catch (PackageManager.NameNotFoundException e) {
      e.printStackTrace();
    }

    String deviceName = "Unknown";

    try {
      BluetoothAdapter myDevice = BluetoothAdapter.getDefaultAdapter();
      deviceName = myDevice.getName();
    } catch(Exception e) {
      e.printStackTrace();
    }

    constants.put("deviceName", deviceName);
    constants.put("systemName", "Android");
    constants.put("systemVersion", Build.VERSION.RELEASE);
    constants.put("model", Build.MODEL);
    constants.put("deviceId", Build.BOARD);
    constants.put("deviceLocale", this.getCurrentLanguage());
    constants.put("uniqueId", Secure.getString(this.reactContext.getContentResolver(), Secure.ANDROID_ID));
    constants.put("systemManufacturer", Build.MANUFACTURER);
    constants.put("bundleId", packageName);
    constants.put("userAgent", System.getProperty("http.agent"));
    return constants;
  }
}

Use getLanguageTag to retrieve device locale