Always use 'en' as the default voice/language.

emscripten/js/demo.js

             opt.value = voice.identifier;
             console.log('Adding voice: ' + opt.text);
             sel.add(opt);
-            if (voice.name === 'default') {
+            if (voice.name === 'en') {
               opt.id = 'default-voice';
               opt.selected = true;
             }

espeak-ng-data/voices/default

-name default
-language en
-gender male
-

src/espeak-ng.c

 	}
 
 	if (voicename[0] == 0)
-		strcpy(voicename, "default");
+		strcpy(voicename, "en");
 
 	result = espeak_ng_SetVoiceByName(voicename);
 	if (result != ENS_OK) {

src/libespeak-ng/voices.c

 			return NULL;
 	} else {
 		if (voicename[0] == 0)
-			strcpy(voicename, "default");
+			strcpy(voicename, "en");
 
 		sprintf(path_voices, "%s%cvoices%c", path_home, PATHSEP, PATHSEP);
 		sprintf(buf, "%s%s", path_voices, voicename); // look in the main voices directory