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.defaultLocale=""

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/DefaultLocaleTest.kt

@@ -0,0 +1,38 @@
+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.BeforeClass
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+
+class DefaultLocaleTest {
+
+  // Work-around to force system property before Paparazzi loads config.
+  // Not a pattern to follow or copy.
+  companion object {
+    @JvmStatic
+    @BeforeClass
+    fun setup() {
+      System.setProperty("app.cash.paparazzi.defaultLocale", "fr-rFR")
+    }
+  }
+
+  @get:Rule
+  val paparazzi = Paparazzi()
+
+  @After
+  fun tearDown() {
+    System.clearProperty("app.cash.paparazzi.defaultLocale")
+  }
+
+  @Test
+  fun `verify system property sets default locale`() {
+    paparazzi.snapshot(
+      view = paparazzi.inflate(R.layout.title_color)
+    )
+  }
+}

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

@@ -79,7 +79,7 @@ public class DeviceConfig(
   public val density: Density = Density.XHIGH,
   public val fontScale: Float = 1f,
   public val layoutDirection: LayoutDirection = LayoutDirection.LTR,
-  public val locale: String? = null,
+  public val locale: String? = detectLocaleProperty(),
   public val ratio: ScreenRatio = ScreenRatio.NOTLONG,
   public val size: ScreenSize = ScreenSize.NORMAL,
   public val keyboard: Keyboard = Keyboard.NOKEY,
@@ -917,5 +917,9 @@ public class DeviceConfig(
 
       return map
     }
+
+    internal fun detectLocaleProperty(): String? =
+      System.getProperty("app.cash.paparazzi.defaultLocale")
+        ?.takeIf { it.isNotEmpty() }
   }
 }