Introduce defaultLocale system property

paparazzi-gradle-plugin/src/test/projects/locale-qualifier/src/gradle.properties

@@ -0,0 +1,2 @@
+android.useAndroidX=true
+app.cash.paparazzi.localeDefault=""

paparazzi-gradle-plugin/src/test/projects/locale-qualifier/src/main/res/values-fr-rFR/strings.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+  <string name="color">Couleur</string>
+</resources>

paparazzi-gradle-plugin/src/test/projects/locale-qualifier/src/test/java/app/cash/paparazzi/plugin/test/DefaultLocaleSnapshotTest.kt

@@ -0,0 +1,40 @@
+package app.cash.paparazzi.plugin.test
+
+import app.cash.paparazzi.Paparazzi
+import com.google.testing.junit.testparameterinjector.TestParameter
+import com.google.testing.junit.testparameterinjector.TestParameterInjector
+import org.junit.After
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(TestParameterInjector::class)
+internal class DefaultLocaleSnapshotTest(
+  @TestParameter private val defaultLocale: SystemPropLocale
+) {
+
+  internal enum class SystemPropLocale(val tag: String?) {
+    Default(null),
+    EN_GB("en-rGB"),
+    FR_FR("fr-rFR")
+  }
+
+  init {
+    System.setProperty("app.cash.paparazzi.localeDefault", defaultLocale.tag.orEmpty())
+  }
+
+  @get:Rule
+  val paparazzi = Paparazzi()
+
+  @After
+  fun tearDown() {
+    System.clearProperty("app.cash.paparazzi.localeDefault")
+  }
+
+  @Test
+  fun `Verify system prop default locale`() {
+    paparazzi.snapshot(
+      view = paparazzi.inflate(R.layout.title_color)
+    )
+  }
+}

paparazzi/src/main/java/app/cash/paparazzi/DeviceConfig.kt

@@ -23,7 +23,6 @@ import com.android.ide.common.resources.configuration.DensityQualifier
 import com.android.ide.common.resources.configuration.FolderConfiguration
 import com.android.ide.common.resources.configuration.KeyboardStateQualifier
 import com.android.ide.common.resources.configuration.LayoutDirectionQualifier
-import com.android.ide.common.resources.configuration.LocaleQualifier
 import com.android.ide.common.resources.configuration.NavigationMethodQualifier
 import com.android.ide.common.resources.configuration.NetworkCodeQualifier
 import com.android.ide.common.resources.configuration.NightModeQualifier
@@ -112,7 +111,7 @@ public class DeviceConfig(
         countryCodeQualifier = CountryCodeQualifier()
         layoutDirectionQualifier = LayoutDirectionQualifier(layoutDirection)
         networkCodeQualifier = NetworkCodeQualifier()
-        localeQualifier = if (locale != null) LocaleQualifier.getQualifier(locale) else LocaleQualifier()
+        localeQualifier = detectLocaleQualifierDefault(locale = locale)
         versionQualifier = VersionQualifier()
         screenRoundQualifier = ScreenRoundQualifier(screenRound)
       }

paparazzi/src/main/java/app/cash/paparazzi/SnapshotHandler.kt

@@ -15,8 +15,10 @@
  */
 package app.cash.paparazzi
 
+import com.android.ide.common.resources.configuration.LocaleQualifier
 import java.awt.image.BufferedImage
 import java.io.Closeable
+import java.util.Locale
 
 public interface SnapshotHandler : Closeable {
   public fun newFrameHandler(snapshot: Snapshot, frameCount: Int, fps: Int): FrameHandler
@@ -28,3 +30,23 @@ public interface SnapshotHandler : Closeable {
 
 public fun detectMaxPercentDifferenceDefault(): Double =
   System.getProperty("app.cash.paparazzi.maxPercentDifferenceDefault")?.toDoubleOrNull() ?: 0.01
+
+/**
+ * Returns a LocaleQualifier for snapshots.
+ *
+ * Uses the provided `locale` if passed in, otherwise falls back to the
+ * `app.cash.paparazzi.localeDefault` system property, and finally the
+ * default `LocaleQualifier()` if neither are available.
+ *
+ * @param locale Optional Locale to use for snapshot tests.
+ * @return LocaleQualifier for snapshot tests.
+ */
+internal fun detectLocaleQualifierDefault(locale: String?): LocaleQualifier {
+  val propLocale = System.getProperty("app.cash.paparazzi.localeDefault")
+    ?.takeIf { it.isNotEmpty() }
+  return when {
+    locale != null -> LocaleQualifier.getQualifier(locale)
+    propLocale != null -> LocaleQualifier.getQualifier(propLocale)
+    else -> LocaleQualifier()
+  }
+}