Add device_language as user property

app/src/main/java/com/hippo/ehviewer/Analytics.java

@@ -18,11 +18,15 @@ package com.hippo.ehviewer;
 
 import android.content.Context;
 import android.os.Bundle;
+import android.text.TextUtils;
 import com.google.firebase.analytics.FirebaseAnalytics;
 import com.hippo.scene.SceneFragment;
+import java.util.Locale;
 
 public final class Analytics {
 
+    private static final String DEVICE_LANGUAGE = "device_language";
+
     private static FirebaseAnalytics analytics;
 
     private Analytics() {}
@@ -30,6 +34,18 @@ public final class Analytics {
     public static void start(Context context) {
         analytics = FirebaseAnalytics.getInstance(context);
         analytics.setUserId(Settings.getUserID());
+
+        Locale locale = Locale.getDefault();
+        String language = locale.getLanguage();
+        if (TextUtils.isEmpty(language)) {
+            language = "none";
+        }
+        String country = locale.getCountry();
+        if (!TextUtils.isEmpty(country)) {
+            language = language + "-" + country;
+        }
+        language = language.toLowerCase();
+        analytics.setUserProperty(DEVICE_LANGUAGE, language);
     }
 
     public static boolean isEnabled() {