Spoken directions use a voice in Gearslip's language

The turns are worded in Gearslip's language, but the speech engine read
them with its default voice, which may speak another one (#70).

gearslip/src/main/java/app/seb3thehacker/gearslip/car/DirectionsVoice.kt

@@ -71,6 +71,9 @@ object DirectionsVoice {
         tts = TextToSpeech(appContext) { status ->
             ttsReady = status == TextToSpeech.SUCCESS
             if (!ttsReady) GearslipLog.w("guidance: text-to-speech didn't start (status $status)")
+            // The turns are worded in Gearslip's language; the engine's default voice may be
+            // another one, and read German words with an English voice (issue #70).
+            else speakIn(appContext?.resources?.configuration?.locales?.get(0))
         }.apply {
             setOnUtteranceProgressListener(object : UtteranceProgressListener() {
                 override fun onStart(utteranceId: String?) {}
@@ -81,6 +84,16 @@ object DirectionsVoice {
         }
     }
 
+    /** Keeps the engine's own voice when it has none for [locale]. */
+    private fun speakIn(locale: java.util.Locale?) {
+        val engine = tts ?: return
+        locale ?: return
+        val result = engine.setLanguage(locale)
+        if (result == TextToSpeech.LANG_MISSING_DATA || result == TextToSpeech.LANG_NOT_SUPPORTED) {
+            GearslipLog.w("guidance: the speech engine has no ${locale.displayLanguage} voice - using its default")
+        }
+    }
+
     /** The car opened its directions sink: start following the map app's turns. */
     fun attach(link: AudioLink, musicIsOurs: () -> Boolean) {
         this.link = link