adding a toggle for forcing english

app/src/main/AndroidManifest.xml

@@ -10,6 +10,7 @@
         tools:node="remove"/>
 
     <application
+        android:name=".App"
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_launcher_name"

app/src/main/kotlin/com/simplemobiletools/calculator/App.kt

@@ -0,0 +1,16 @@
+package com.simplemobiletools.calculator
+
+import android.app.Application
+import com.simplemobiletools.calculator.extensions.config
+import java.util.*
+
+class App : Application() {
+    override fun onCreate() {
+        super.onCreate()
+        if (config.useEnglish) {
+            val conf = resources.configuration
+            conf.locale = Locale.ENGLISH
+            resources.updateConfiguration(conf, resources.displayMetrics)
+        }
+    }
+}

app/src/main/kotlin/com/simplemobiletools/calculator/activities/MainActivity.kt

@@ -22,6 +22,7 @@ import me.grantland.widget.AutofitHelper
 class MainActivity : SimpleActivity(), Calculator {
     private var storedTextColor = 0
     private var vibrateOnButtonPress = true
+    private var storedUseEnglish = false
 
     lateinit var calc: CalculatorImpl
 
@@ -53,10 +54,16 @@ class MainActivity : SimpleActivity(), Calculator {
 
         AutofitHelper.create(result)
         AutofitHelper.create(formula)
+        storeStateVariables()
     }
 
     override fun onResume() {
         super.onResume()
+        if (storedUseEnglish != config.useEnglish) {
+            restartActivity()
+            return
+        }
+
         if (storedTextColor != config.textColor) {
             updateViewColors(calculator_holder, config.textColor)
         }
@@ -65,7 +72,7 @@ class MainActivity : SimpleActivity(), Calculator {
 
     override fun onPause() {
         super.onPause()
-        storedTextColor = config.textColor
+        storeStateVariables()
     }
 
     override fun onCreateOptionsMenu(menu: Menu): Boolean {
@@ -82,6 +89,13 @@ class MainActivity : SimpleActivity(), Calculator {
         return true
     }
 
+    private fun storeStateVariables() {
+        config.apply {
+            storedTextColor = textColor
+            storedUseEnglish = useEnglish
+        }
+    }
+
     private fun checkHaptic(view: View) {
         if (vibrateOnButtonPress) {
             view.performHapticFeedback()

app/src/main/kotlin/com/simplemobiletools/calculator/activities/SettingsActivity.kt

@@ -3,8 +3,11 @@ package com.simplemobiletools.calculator.activities
 import android.os.Bundle
 import com.simplemobiletools.calculator.R
 import com.simplemobiletools.calculator.extensions.config
+import com.simplemobiletools.commons.extensions.beVisibleIf
 import com.simplemobiletools.commons.extensions.updateTextColors
+import com.simplemobiletools.commons.extensions.useEnglishToggled
 import kotlinx.android.synthetic.main.activity_settings.*
+import java.util.*
 
 class SettingsActivity : SimpleActivity() {
 
@@ -17,6 +20,7 @@ class SettingsActivity : SimpleActivity() {
         super.onResume()
 
         setupCustomizeColors()
+        setupUseEnglish()
         setupVibrate()
         updateTextColors(settings_scrollview)
     }
@@ -27,6 +31,16 @@ class SettingsActivity : SimpleActivity() {
         }
     }
 
+    private fun setupUseEnglish() {
+        settings_use_english_holder.beVisibleIf(config.wasUseEnglishToggled || Locale.getDefault().language != "en")
+        settings_use_english.isChecked = config.useEnglish
+        settings_use_english_holder.setOnClickListener {
+            settings_use_english.toggle()
+            config.useEnglish = settings_use_english.isChecked
+            useEnglishToggled()
+        }
+    }
+
     private fun setupVibrate() {
         settings_vibrate.isChecked = config.vibrateOnButtonPress
         settings_vibrate_holder.setOnClickListener {

app/src/main/res/layout/activity_settings.xml

@@ -30,6 +30,25 @@
 
         </RelativeLayout>
 
+        <RelativeLayout
+            android:id="@+id/settings_use_english_holder"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:layout_marginTop="@dimen/medium_margin"
+            android:background="?attr/selectableItemBackground"
+            android:padding="@dimen/activity_margin">
+
+            <com.simplemobiletools.commons.views.MySwitchCompat
+                android:id="@+id/settings_use_english"
+                android:layout_width="match_parent"
+                android:layout_height="wrap_content"
+                android:background="@null"
+                android:clickable="false"
+                android:paddingLeft="@dimen/medium_margin"
+                android:paddingStart="@dimen/medium_margin"
+                android:text="@string/use_english_language"/>
+
+        </RelativeLayout>
 
         <RelativeLayout
             android:id="@+id/settings_vibrate_holder"