Bug 42376: Pass the locale list when constructing l10n in datetimebox

The datetime input is inconsistent with other inputs when using spoof
English: its placeholder is not translated, unlike the default values
and texts of all the other inputs.

toolkit/content/widgets/datetimebox.js

@@ -22,7 +22,9 @@ this.DateTimeBoxWidget = class {
     // createEditFieldAndAppend() call.
     this.l10n = new this.window.DOMLocalization(
       ["toolkit/global/datetimebox.ftl"],
-      /* aSync = */ true
+      /* aSync = */ true,
+      undefined,
+      this.window.getWebExposedLocales()
     );
   }